I've always wondered how cost effective Flesh Golems were, considering the amount of units you'd need to sacrifice to bring its strength up. Anyone manage to come up with a reasonable way to use them?
Graft a hawk onto them
FLYING FLESH GOLEMS.
But in general, it's like build your own hero. You can give them almost anything aside from spells. No matter how awesome he is, valin phanuel can never fly. And any hero is eventually going to suffer diminishing returns as it takes more and more xp to level up.
I find that flesh golems are especially good as calabim. Make some new vampires, and have them feast their way through the first few cheap levels. Promote one with combat I-V, one with Drill _IV and blitz, one with shock, cover, formation, scourge, etc. One with magic resistance, and all the elemental resistances. And then just fuse them all together into one golem, handily provided by a vampire lord.
The golems can be vampiric too, although the extra healing is the only real benefit they get. Golems can't level up so feating is useless.
Another good thing to do, is cast mutate on stacks of warriors. And fuse the good mutations with your golem. It can get cannibalize, heroic strength/defence, and such things like that.
Oh, and ANOTHER good thing to do, is playing as balseraphs. Send your mimics out across the globe, collecting rare and exotic promotions like Dexterous, Woodsman/Guerilla II, etc. And them fuse them all with your golem, for one of the most unique and varied units you'll ever see.