Through my experience as an Eldar Epic and Epic40K player, I have developed some new tricks or have acommodated old ones. I have mostly played on 2000 pts in Epic40K.
In many battles against Chaos I have continually faced the problem of a high-assault mobs of the opponent rolling down the field towards my thin line of precious Eldar troops. In response to that menace, I developed the ideas of combining the high speed of my jump-packed aspects and my bikes.
Suppose you are facing two slow demon det of about 15 units each and you have a det of 10 jump-packed aspects and two det of 10 jetbikes. Put both of your dets on assault. Win initiative in normal movement. In your normal move, pull back the aspects to just outside 30cm away from the demons, but retain direct apposition with them. Pull the bikes away to the left and to the right of the aspects at about the same distance - barely outside 30cm away from the demons.
At this point, your line should look like a crescent open towards the enemy, where the aspects are in the base, the jetbikes are in the horns, and the demons are between the horns. If Chaos wins initiative in the assault phase, he can get a little bit closer to you, but cannot engage, since you are outside 30cm, and the demons cannot double their 15cm unless they can engage. In your assault, hit demons frontally with the aspects. Try to two-on-one the demons. You do not need casualties from hits; all you need is to to win the assault. Remember, a demon supports with 1, but fights with 4.
The bikes engage with one or two bikes of each det, so that you can count most bikes as support. The unengaging bikes position themselves about 10cm away from the demons so that they can support and prevent retreat. Ideally, you should be able to close the crescent into a circle, whence the name - encirclement. Winning the assault should not be difficult when you support with so many bikes and apply two-on-one. Then everyone within the circle of death is lost. The added bonus is the ability to score all blastmarkers you have inflicted immediately denying the opponent the chance to recover them. If you have doubts that some enemy stands can manage to get outside the 15cm gun-down, choose them as the casualties in the assault.
If this happens next to a table edge, you will need only one det of bikes.
If you lose initiative in normal movement and you are made to move first, you just need to pull back keeping in mind the most the demons can move on assault. If they move their most, very little changes; if they don't, you still win, since your superior firepower will have more turns to shoot before the demon wave hits.
A fact to remember is that sometimes it will be beneficial to engage with more bikes at assault penalty, if that will allow other bikes to move further and close the circle.
The example here is against Chaos, since the trick was originally developed against Chaos. However, the general idea and geometry are applicable against all other armies.
Encirclement is a fine thing, but you should not expect to succeed for long singly by it, especially if you have such worthy opponents as the ones in my gaming group.
An expensive but effective countermeasure is to have a long solid line of troops stretching from one end of the board to the other, since then the bike horns cannot flank the enemy and close the circle. The aspects can hit a segment of the line, but its neighbours can encircle the aspects to avenge their comrades.
An easy solution to that problem presents itself against high-assault zero-firepower units like the demons. By careful positioning and choosing your target of jump-packed aspect hit, you can assault and break the central det and firefight and break its neighbours - the aspects do have a gun. That makes sure the demons do not counterattack in the same turn. Iterate next turn.
This little thing will only work against a dense demon line of single-det depth. A Chaos player quickly learns to echelon his wave. You hit the front echelon but the second demon line will jump out from in-depth to stomp your aspects. Additionally, Chaos will start taking engines/silver towers/cultists to protect himself from firefights.
Thus I have become much more careful with the aspects and I have developed the containment trick. It involves two different strategies applying to different constitution of the enemy echeloned wave.
Suppose the enemy wave contains only two lines of demons. Then I do the Glorious Sacrifice of the Lonely Bike. Take a det of 10 bikes. Put them on assault. In the assault phase, a lonely bike separates from the group, jumps right in front of the demon line guns-ablaze, and firefights the entire demon line back. The second line comes in and vengefully rips the bike to pieces, which results in one BM for you and a broken det, which retreats a little bit to recover BM and rally. To recapitulate, Chaos loses its entire assault move, since the two lines have effectively exchanged place at the cost of one bike per turn. That buys more time for your shooty units. Just do not roll snake-eyes for BM/rally.
Suppose the enemy wave contains a sandwich - antifirefight line between two demon lines. Then hit the front demons with the aspects. Put the bikes right behind the aspects and engage with one or two bikes so that all bikes support. That breaks and drives away the front demons. Together bikes and aspects should win the firefight with the antifirefight line, break it and drive it back. Now if you are lucky, the second demon line may be close enough to be firefought and broken too. If not, they will jump on your aspects, kill some and break them, but then they will lose the firefight with the bikes, who are assault-shielded by the aspects. To recapitulate, you kill a bunch of demons, you break the entire wave and drive it back 15cm, at the expense of several aspects. That does buy a lot of time for your shooty units.
In many cases, against slow armies which fail to form an edge-to-edge assault wave, it pays to sacrifice a turn of shooting to redeploy your entire shooty defense line to the other side of the map by putting most units on march. That buys you several more turns of shooting, before the closeness of the enemy assault wave necessitates a new redeployment. You simply move in a big circle, whereas the opposing army mostly tries to follow you along a path in the shape of an expanding spiral concentric with your circle. If you manage to gun the down or reduce them to assault-manageable amount before the spiral crosses the circle, you win.
The Eldar lost all barrage weapons. Fortunately, they seem to have the best disrupters in the game. Use Night Spinners on dets you want to pin down, so that they do not move or go on assault; on war engines, so that you shut down death rays and megacannons; on enemy artillery, so that you shut down artillery pieces; on dets you are going to wipe out, so that you score the BM; on shooty dets, so that they do not go on overwatch.
This works against armies which have trouble taking ground. Distribute a det of 10 scouts in a 60cm-radius semicircle centered at the HQ. If skillfully done, that should ensure 4 to 5 capture-and-holds, with all scouts still in command. The enemy has to divert precious firepower to the scouts to pick them out one by one, or let you soak up morale every turn.
If you play enough, you will discover that some units have considerable psychologically terrorizing effect on the opponent. In many cases, what they threaten to do and the opponents' panicky countermeasures are much more valuable than what the units can really do. Good examples of Eldar terrorizers are the Warlock Titan, the jump-packed aspects, and the night spinners. Exploit that psychological factor to the full by preserving such units until as late in the battle as possible.
Many armies need to advance in a strict formation to protect themselves against tricks like encirclement or containment. Use that to your advantage. Let him advance, while shooting him from afar and bombing him. The opponent must make a choice - he has to advance cohesively, i.e. in formation, at the speed of the slowest unit, or he has to lose cohesion to move at full speed. You win in both cases - you either shoot/bomb him longer or take advantage of the exposed formation weaknesses.
The Eldar army is a sum of fast, high-quality, expensive, highly-specialized units. This predestines that the backbone of Eldar strategy is severe customization, profound planning, high discipline, and very flexible and mobile field behaviour. The Eldar come unexpectedly from nowhere, deliver a backsnapping lightning attack, and retreat as fast as they have arrived before any retaliation.
In my opinion, an Eldar general must feel and act like a raider and a terrorizer, rather than an old-school field-bureaucrat, a religious zealot, or a bloodthirsty beast. The minute an Eldar general gets any stupid ideas into his head, e.g. staunchly holding his ground, gallantly engaging in an artillery duel, or heroically charging with his handful of aspects into the enemy hordes, is a minute of great grief and bloody levy for his Craftworld.
The Eldar strengths: high speed, good firepower, decent cannon range, profuse anti-tank, best space fleet, best jump-packed assault infantry, best disrupters, a lot of skimmers, a lot of psykers, good walkers, indestructible titans if the dice are with you.
The Eldar weaknesses: small infantry and bike range, bad tank armour, bad war engine armour, small damage capacity, automatic titan criticals, lame and expensive command units, ridiculous flak, no affordable transports, annoying det structure, seriously overpriced tanks, no drop pods, wipe-out penalty, small dets, no barrage weapons, no megacannons, no death rays.
Enemy-specific musings follow.
Jump-packed aspects are your terror troops. Bikes encircle and firefight; night spinners pin down the demons. Anti-tank on support platforms and scorpions to kill that annoying supreme commander that keeps prodding the chaos horde forward, the engines and silver towers that protect the demons from firefights, and those scary CSM land raiders. Nightwings must instill Chaos flyers with unholy awe. Phoenices allow you to play the waiting game and smack the Chaos artillery. Dark Reaper Exarch is Chaos's nightmare - long-range, profuse firepower and high assault in one. Be a sport - never ever take battletitans - Chaos cannot kill them without many land raiders. I took a Warlock once and felt terrible afterwards.
The strategy is: give ground slowly but steadily; be ready to sacrifice bikes now and then; preserve the terror troops as much as possible but sacrifice them if you can save dark reapers, scorpions, and artillery; buy your time at the expense of containment troops. Be ready for redeployment if need be. Try to deny double assault movement to the enemy as much as possible. If you think your nightwings have splashed enough doomwings in the past for Chaos to get the message, try gambling - take only bombers some day. Ignore cultists, beastmen, and CSM bikes as much as you can.
Jump-pack terror and space fleet are valid as usual. Dreadnoughts are great since IG does not have much anti-tank and has bad assault. War Walkers are downsized dreadnoughts. Definitely a Warlock Titan as a deterrent against IG titans and tanks. Take falcons only as means to get walkers. Night spinners are a gamble, since siege artillery has bigger range. Maximize armour and range. IG cannot take ground - assign scouts to morale harvesting.
The strategy is: hope that your maximized armour and the IG anti-tank deficiency will decisively increase your longevity. Hope that the titan does not fail his holo save too often. Load up on 45cm ranges - shoot those 3+ guardsmen/ 4+ rough riders dead before they close in. When they are too close, stomp them with the dreadnoughts and the aspects. Bomb his artillery into oblivion. Pulsarize the leman russ and anti-tank support weapons. Keep pressing him with your aspects - there is no point in holding back since his artillery range is too big and you jump on him from outside his infantry range. You can really push him off the board if you try hard enough. And, most importantly, for God's sake, spread out! He will find his barrage artillery next to useless when he can get only one unit under a template - the Eldar are a few, they can do it. If you gamble with night spinners, mostly try to shut down/keep from overwatch artillery pieces and leman russ; if he is silly enough to arm a titan with death rays and megacannons only, shut him down completely - I bet that is a lesson he will not forget.
Jump-pack terror and space fleet are valid as usual. Night spinners are nice unless you play with a cheeseball who takes IG artillery companies. Definitely a Warlock titan. Dark reapers seem a good idea. Scorpions are very good in skillful hands. Armour is pointless against the land raiders. Bikes may be useful for encirclement and defense if your opponent likes the ravenwing but hates terminators. I like including anti-tank support platforms in the night spinner det to discourage drop pods with assaults. Stress nightwings heavily to splash those thunderhawks. Do not bother with bombers unless you feel lucky.
The strategy is: make sure those land raiders are silenced by the Warlock Titan and the scorpions. Once the land raiders are down, move in with the titan and the aspects and stomp the marines det by det. If the land raiders get too close to shoot back, do not be ashamed to turn tail and run even with the titan; you can still shoot backwards due to good firing arcs on both titan and scorpions. Dark reapers seem an excellent choice since anti-tank does not have advantage against them, they are cheaper than tanks, and they have 45cm to pick out the tacticals and terminators safely. Anti-tank support platforms in the night spinner det will spoil the fun of the assaults any day of the week. Scorpions require cold blood and skill to use since two land raiders will pop one every turn on overwatch if you venture within range. Spread out to make whirlwinds useless. Hold back the aspects until the land raiders are down and the assaults have dropped.
Squats are a dull boring army, so you need to play dully and boringly to win against them. Just pick two Warlock Titans, thirty jump-packed aspects, a scout det and two phoenix dets. Scouts do morale harvesting. Phoenices bomb. Titans advance, make holo saves, pulsarize around, and recover blastmarkers. Aspects hold back out of range of megacannons until the titans are about to hit the enemy line. Then the aspects join the festivities. You will ALWAYS stomp them as long as the dice are with you. You will always get vaped if you start rolling 1's. This is so dull it makes me weep! You might just as well roll 30d6 at the beginning of the game. If you roll less than five 1's, you claim victory. If you roll above five 1's, you concede the game. I must stop before I die with dehydration due to weeping.
Titans, jump packs, anti-tank, and night spinners are your friends. Use bikes for encirclement. Phoenices and dark reapers are next to useless against such armour. Nightwings may blow up the harridan but the gargoyles will victimize them. Better try the Eldar lamest new unit - the Fire Prism - neither a prism cannon nor a firestorm, but a little bit of both, just too little though and way too expensive. In this case though, they seem useful since they will flak when the bombing comes and will anti-tank in the other turns. Most obviously, anti-tank the synapse creatures. You might consider distortion cannon, since it causes an automatic critical and no save is allowed if you hit. If you manage to protect the scorpions, they can be worth it. Tyranids may be susceptible to disruption of cohesion depending on how many mycetic spore dets and assault spawns they have.
The Orks are a 45 cm range army. They pretty much have to take gretchin living shields to protect far more valuable units like shooty boyz or scarboyz. This means firepower will not do very well, because while you are killing gretchin, the shooty boyz will be killing your dark reapers and falcons. This makes the orks a hard army to deal with when you lead Eldar. Still, once you recognize this, victory is within reach.
Load up on 60 cm ranges through titans and scorpions. Anti-tank means you can pick out the choicy targets like the shooty boyz and pulsa rokkits while ignoring the gretchin shield and staying out of the deadly 45cm. They of course will try to creep up on you, but you can pin them down with night spinners. When 30+ stand detachments freeze under fire, it is time for the ork warlord to start worrying.
Simultanteously, you have to have the jump-packed terrorizers to be used as described above. Also, since you are going for a slow win anyway, it is wise to get a wing or two of phoenix bombers. Gretchin cannot protect from all directions, so attack a formation from the exposed end and let those fusion bombs fly - ork armor sucks, so it will be a bloody massacre.
Don't forget to manoeuvre and give ground steadily. The more turns you get to shoot them, the fewer will be left when you will be forced to counterattack with jump-packs and titans into close combat.
Depending on the orky favorite army composition, you might have to tailor these general ideas. For example, if your opponent is a battlewagons guy, you might want to anti-tank those first, so that you quickly cut down the opponent's manoeuverability. If he likes fighter/bombers, just spread out because those things use barrages. If he is a bikes and buggies freak, consider taking out the bikes with firepower, your bikes, or jump-packs; here firepower will work because bikes are too fast to be shielded with gretchin.
The Eldar are a connosseur's army, so your opponent is likely to make numerous mistakes in army composition or he may not be sufficiently active or inventive on the battlefield. His punishment will be swift if you just utilize those errors. If he is more experienced than you, chances are you are screwed. Learn his tricks.