As noted, it is difficult to balance the game when the number of players could change.
At the end, when you started bringing the mobs in multiple groups, it game us the option to nickle and dime them down. Otherwise, the Trooper would handle them with a single roll.
It might make sense to have the Trooper dispatch three in a round rather than max.
The "Potent" enemy would have changed the odds a bit but in some battles were were treating 4's and 5's as 6's .... would not have mattered much.
But, a two player event would have been different.
Perhaps, add a monster option where everyone has to hit it simultaneously to defeat it. That would make a 4 player session tougher ...
I don't see an Empath taking out a group of baddies which seems more common than an a series of inconvenient truths.