Did you use your peers on any of the other levels?? That way they should be all up around level 6 or something, they should be capable of holding their own. Make good use of the spears that are jabbed in the ground and the fire-things you can turn over, that way you can instantly clean up a huge group of enemies. Plus divide your attention between your peers and as soon as a mini-boss enters the arena and attacks one of the others make sure you're there they make the most damage(against the normal enemies they can stand their ground for quite some time). Keep running around knocking over fire-things when there aren't any mini-bosses around.
Too take a mini-boss out make sure you hit him a few times with the single hard attack in order too knock his armor-pieces off and then just combo his ass out of there.
Enjoy the ride, cause it's gonna be one hell of a fight
That level got my adrenaline pumping soo hard!