Omega's not too bad, GuGalle and Artanis can be wiped out just by massing Hydralisks, and Mengsk can then be wiped out by Muta/Guardian.
To Slay the Beast is the only really hard mission, I think, and it's mostly because of that river of lava. Bastards keep killing (and Blinding) my Overlords when I try to ferry my troops across, and any attempt at establishing a Hatchery for Nydus is annoying as hell. Even mass air, aside from being ridiculously expensive, takes a lot of damage from Devourer/Valkyrie and Darkswarmed ground units. Still, that usually works well enough.
I've just finished True Colors. I always have the most ignoble death in store for poor Fenix, he's one of the only reasons I ever research Spawn Broodlings.








