Please please please, if you're going to make the AI ridiculously hard, include a big chaos factor. What
Scaredgirl suggested sounds okay, but will still make the AI difficult. Certain "bugs" like having the AI target your bonewall made FGs beatable.
I played a gravity shield against osiris, and he didn't devour his own scarabs to drop their health below 5. In fact, he spawned more scarabs O.o
My purple nymph leisurely (friggin' turtle shield...) made his scarabs work for me, and them my black one turned them to the dark side of the force. I targetted his strongest scarabs in order, and I noticed that when he finally could, he did devour the affected critters, which I think is a new feature, but he could have gotten around my gravity shield early on and killed me before I got my nymphs out (4 3-attack scarabs, the fifth having too high a defense after devouring, dealign 12 damage, would have killed me in 6 turns with my already lowered health).
Decay played ecclipse when I had 5 vampires out and he had no creatures. He didn't play anything for another several turns, and meanwhile my adrenalined vampires kicked his ass O.o
Not that I mind the AI giving me a hand, but that's just stupid.
I suggest that you make FGs easier without making them look stupid. Make them not do things some of the time rather than make stupid decisions.