Please consider making horde mode on twins arena.
Make stronger hordes with each wave ( just composing them by whatever algorithm ).
Difficulty selector determines how many waves will you skip before start.
Each 5th wave has boss +1. So on 15 wave you have 3 bosses + crushers, ragers and horde ofc.
Make portrait frame with max reached wave on it.
Maybe it will even become your endgame.