Guardian
The Guardian is a heavily armed monster which attacks any fleet arriving at the Orion system. The player who kills it receives 4 technologies which are mostly inaccessible through conventional research (listed at the bottom), gets first dibs at a rather nice planet, and gets a free battleship, which if he played fast enough, should be able to whack the other opponents' fleets handily. Under some circumstances, a good player should be able to finish off all of the opponents before acquiring the ability to kill the Guardian, but usually, a Guardian-kill can be considered a win.
In the first versions of MoO2, the Gyro destabilizer was a cost-effective weapon. For many races, the fastest way to kill the Guardian was to use a few battleships or about 30 frigates filled with gyro destabilizers to spin the Guardian to death. Then, the folks at Microprose decided that that was too easy, or that it was negating the intended use of the destabilizer - that of turning a ship so its forward-facing weapons couldn't shoot (at least I assume that was the intended use). Now gyro destabilizers take up more space and do less damage, so while you can still kill the Guardian with them, there are much better ways now. They are detailed below.
The Minimal Guardian Fleet
25 Battleships with phasors can destroy the Orion guardian (shield
piercing is useful against the guardian).
-- Colin Glassey
In this sense, the considerations for "minimal" are with respect to research and production time. This allows some deviation depending on your research/production situation. As it turns out, the Guardian can be killed with four frigates and two cruisers, and no research beyond fast missile racks, zortrium armor, and emissions guidance system. Since the Guardian has hard shields, the shield piercing attribute of some weapons is completely useless.
First cruiser has:
Second cruiser has:
Frigates have:
First turn: The Guardian will advance and fire on one of the cruisers. If it explodes, then it's just not your lucky day - there is a low but non-zero chance that the death rays will obliterate the cruiser's engine. Otherwise, follow the directions here. The frigates should try to clump near where the cruisers are, moving diagonally but forward two spaces, then turning toward the Guardian and firing. The first cruiser should move forward two spaces, fire both racks of ARM missiles, and then both racks of MIRV. The other cruiser should move forward two spaces, fire the ARM then the MIRV missiles, move forward one more space, and fire the ECCM ARM FST missiles. If the unhurt cruiser was not directly in front of the Guardian in the beginning of combat, it should make its way in that direction after firing. This is essential.
Second turn: The Guardian will react by moving forward, killing the wounded cruiser and perhaps some frigates and ARM missiles. The torpedos will disappear, as the Guardian apparently lacks Dauntless Guidance System technology. The remaining frigates should then sit still or move toward the center while still allowing the Guardian room for forward movement (so it doesn't turn to fire and inadvertently absorb the missile damage on more than one shield face). The Guardian will charge, trigger its spatial compressor on the ECCM ARM FST missiles, and continue to plow into the pile of missiles. The MIRV missiles should clear the shield, and most likely, the EMG missiles will blow the engine. If you want to be more sure about the kill, add another frigate into the mix or research some scanner technology.
I have personally killed the Guardian on turn 98 with this fleet in a pre-warp game using a democracy-lithovore-artifacts homeworld race. Several other races can pull this off in comparable time, at least by turn 120. When I get around to updating my races page, I'll fill in the details. The Guardian-kill can obviously be done more quickly in an advanced game given the proper parameters. In particular, a subterranean creative race in a 2-player huge universe can often finish off the Guardian before turn 20.
Other Considerations:
I'm pretty sure this is the minimal practical fleet as far as tech goes. If you don't have Zortrium Armor, then you don't have MIRV merculites, so it is much more difficult to take down the front shield and destroy the engine. Nuclear missiles have the MIRV option before Zortrium, but they do zero damage to the Guardian regardless of modification because the 8 damage per warhead is reduced by 5 by the class V shield and by another 3 by the hard shields. Fast missile racks keep the fleet small, and EMG is necessary for any kind of graceful kill. This fleet has slightly better than even chances of killing the Guardian, but the nice quality of it is that the addition of extra frigates is easy and rapidly increases the chance of blowing the engine. If you are not satisfied with a 50% chance and instead want say, 95% confidence, you will need to put some redundancy in the fleet to keep an unlucky engine hit from ruining your day. Four cruisers will suffice, but there are probably more efficient configurations. I just don't bother trying them out.
If you have good scanner technology (i.e. Neutron), you should be able to clear the shields with fewer missiles, so if you're the gambling type, you can drop some frigates from the fleet and put an EMG missile into one of the cruisers, firing it after the MIRV missiles. If you have serious command point difficulties, you can kill the Guardian with a single kamikaze battleship with the following:
While this ship requires less production than a the cruiser fleet above, a Guardian-kill is strictly less likely: note that at least 4 EMG warheads need to make it through the lightning field and ECM to blow the engine, and 21 warheads need to impact the shield to clear it, so given that only 22 normal MIRV's (88 warheads) and 4 EMG's (16 warheads) are fired, the shield might not be cleared and the engine might not be blown. In fact, without neutron scanner technology, your chances are under 40%. Furthermore, the flexibility of the fleet with regard to minor expansion is worse, because if you decide to add some frigates to the fleet, the Guardian's warped decision tree will cause it to fire on those first, so you need to add at least three to make any kind of difference in expected outcome. Finally, this fleet will generally lose all of its ships, while the cruiser fleet will not, and it is often convenient to guard Orion from computer colonization with a cruiser and perhaps a frigate while whacking enemies with the Avenger (you have other concerns against a human opponent).
Loknar's Tech:
Players on Intel-compatible machines will receive death ray technology every time and three random techs, while Macintosh users will receive a random selection of four. In addition to the technology listed above, you can get some unannounced "freebie" techs which range from dumb to very useful (I've received Sub Space Teleporter more than once).