Its a case of you trying too hard to do an easy mission :\
How I did it with Prot Monk Hero, Koss, and Nuker, 2 henchie monks, illusion hench, and Devona: took like 5 minutes .
1)I went to middle and let the 2-3 groups of Margonite come from the side and kill them.
2) Took Middle.
3)Went off to rift on one side, killed the margonites there, Blas guy would be trying to take the middle at the same time, kill him and energies before he fully takes it.
4) After Blas and energies are dead, quickly go back and take the rift you cleared eariler, just leave the heros in the middle to pwn the Blas and energies while you doing this, you dont need them anyway (max +4 degen on taking it anyways)
5) Repeat at each rift. It gets REALLY easy after the first or second rift because he spawns alot at the rift you are fighting at, so you can just kill him there
If you can do without your heros in the Margonite battles, just leave them in the middle at the whole time on Blas/energy killing patrol while you take rifts with henchies and yourself. That works too, and probably faster.