It first epic level is hte hardest. It is a combination of getting the right gems (poison and armor - green and purple) and having enough towers so you can move the gems along the path as well as water trenches to slow him down. THen also enough Mana so that he can go round twice.
I think I defeated the epic mosnter the first time with a few Grade 6 and grade 5 gems.
Don't overcombine gems. I now never have more than two color gems. If you have more than 3 colors, the special abilities of the gems are lost. Even at 3 colors, they only function at 60%.