Guardians: Gravity, Zombie and Artillery. (need to destroy these or u die)
Fleet Ships: Spire Gravity Drainer, Spire Stealth Battleship, Zombards, Snipers, Vampire Claws, DoomShrooms (surprisingly nasty when used by the AI), Shield Bearers, EtherJet (Thief AI..(800 EtherJets=bye bye fleet)) and Spire Maws but I usually disabled swallowers.
AI types: Hard: Raid Engine, Mad Bomber, Zenith Descendant, One-Way Doormaster
Hard and annoying: Thief, Technologist Parasite, Stealth Master
Impossible combos: Raid Engine/One-Way Doormaster, Mad Bomber/Support Corps
Hard combos: Speed Racer/Tag Teamer, Experimentalist/Support Corps, Fortress Baron/Shield Ninny(or Support Corps)
(of course something like double Raid Engine or Mad Bomber would be retardedly hard so no need to list it herp a derp)
Also I agree with you about the Support Corps. It seems to me that Support Corps is underrated. It can be surprisingly nasty.