Pull them into the death plane with the orb
RNG time! We now have to wait until the Scavenger is at the edge of the arena. There are two methods to quick-kill the Scavenger from here:
The Scavenger does their "jump-downthrust" attack at the edge of the arena: grapple them as they hit the ground.
If the Scavenger runs to the edge of the arena, you must immediately ice-beam them, wait for the ice to decay, and grapple them into the pit just before/as the ice is breaking.
Because the ice-decay explosion will knock you back, you must move a bit away from the ledge before grappling the boss. But if you walk back too far, you will grapple the Scavenger to your platform, and the run is basically over.
It is very tricky to get into the proper position to execute the ice-grapple, and there are various ways to do so. You'll have to experiment to find what works for you. This is how some do it:
If done correctly (regardless of method), Scav will fall into the pit and die. If you have any mana left, a quick wand shot or orb grapple could make them die faster.