There are two things you need to do as a workaround. Send your companions on missions to keep them away and also jump off the arena as soon as combat starts and then they will attack each other. When there are only a couple left you can go back up and finish them off and complete the quest.
I like the Merc. It's got great AOE abilities and also single-target....can be tricky though when you get those mobs that just aren't quite close enough to AOE! Being able to queue as both DPS and Healer is cool.