i've set my multiboxer addon to follow/accept/assist everything from "focus"
Then, on my PiP keys i have a macro to set the focus to whicheverpip toon i PiP in.

And all dmg macros have a /assit focus before the /cast line

Like that, no matter who is alive/dead, i just have to PiP into the screen of a live guy and he becomes the main automatically
dunno if this helps