When you zone in from anything(in this case a town portal) you get 20 second grace period where you are immune to damage.
the trick here is to have the invulnerable person TANKING and have dots,ranged,traps etc kill the boss.
this can be done with 2 people or more people but the concept is the same.. with more party members you can actually block the mob in a corner for 100% safety
can be repeated as many times as you have town portal scrolls..
this is extremely useful for turbo/lethal brutus in a race and i'm sure can be applied to other bosses..