This sounds tricky as if you've played this level much you know the sack gathering finale is a pain. This solution will be more of a suggestion, I'm certain you can do it other ways.
The strategy my group employed for the finale was to essentially take it sack by sack, slowly and carefully and to let people die and respawn if they got downed or into grey health.
Players dying during the finale respawn in the room where you cut the rope to start the finale. It is easy to defend so you can let a player die and then regroup into the room while they respawn. It is wise to have at least two players alive at a time so that it doesn't get ended by an unlucky special rat combo.
Potion spreading potions of speed will give you an extra edge here too, so try to find some during the level.