I've read other random posts about people saying they've killed an epic before as a creature but it doesn't really seem possible. Maybe if you like...use spit and then sneak away and keep doing that until it dies. It would take nearly an eternity to accomplish but it might work.
I had 1 rogue creature with me, and two of my own; had level 5 spit. Just spit and run, and when he gets low enough, go in and bite/claw him to death. I've done it to two or three of them at this point; it does take a LONG time, and as far as I know, you don't gain anything from it (other than an achievement the first time you do it.) It's still a pretty nice personal accomplishment.
Have killed epic since day first got spore it easy just use the trainer & you can kill epic with just one creature. but be careful cause in 1.0 you could get stuck a lot and would save then reload after killing it.
one method I have noticed and have been able to duplicate is using spit not sure if it helps but I put extra spit parts on. get his attention and find a large tree or rock have it on one side and you directly on the other side positioning is key as it may sometime try to go around activate instant cooldown and spam that spit button. I do this solo take a few minutes but easy.
NOTE: only need to do this if it has graspers (hands) if it has no hand just to bite/slash it with helth feature on as without hands it can't throw you.