LOl, I once tried to kill the huge demon boss (the one with the sword), but it was impossible, the lord of fire was stuck there and he didnt want to keep on going, but it didnt really matter, because he took 15% of his HP, but another 15 min after, he didnt lost any HP afterall.
a time consuming way but easy, and you dont even need to be present while you do this is to:
summon a tower that shoots arrows which will take care of most running units that will attack you.
Then use succubus, move them to just outside the "attack reach" they will be killed several times, but they will also shoot alot at this guy and since the will spawn infinite you just let them do the work, when he is down you go in with all your units.
All i do is check in and move them back at times, cause if one is getting ahead, all will die when he starts swinging. and I use spells for attack and rage. to fasten everything up. works in normal hard and epic.
Have one battle that is last in epic. at the moment im not sure how to do it and why im here lurking for some good ideas, know what im going to try next