Send your Void walkers in first (obviously), but have them attack your first and last target, then have the one attacking the last target switch to the one you want to kill second after a few seconds of building aggro on the last one. I hope that wasn't too confusing...You can switch the order up anyway you want, and it may be easiest to have your Main's VWer tank the 1st and 3rd if it's easier to control, but you just need to build up some threat on them all. It's easiest if you can just send them at the troggs where they stand, because then they will get initial aggro anyways. If you have to pull them away, it could get harder.