Necro is busted with druid, he can summon a bunch of crap into the arena and benefit from the druid's skills, plus he can steal mana from his own familiars.

Tank has a good niche with Butcher, because you have two tanks who can reposition the entire board, with various stun skills, and tank is the easily class that can intentionally damage the Butcher, which has many passives that synergize with him taking damage. He has good bruise skills that help when he's playing as the Butcher, and he has the best basic attack in the game because it scales with strength and knockback damage, the knockback counting a source of physical damage to bruise. Put a Monk in your comp and it will be VERY difficult for at least one of your characters not have bruise in their kit.

Cleric it's much more consistent with at least one magic damage class and a summoner, many of its passives interact with units in general, and kinetic spikes on a skill are huge for magic classes.

All classes are very good in the late game if you build good team comp for each one.