Have one dominate proof hero. Put hero in back most row for 3rd boss turn. Boss will always try to dominate a random hero (Taunt doesn’t work) in the back most row. That could be the 2nd row if no one is in the 3rd. Tommus, Jarlaxe, etc. Tom for example was better at keeping my guys alive, but could with counterattack get a few hits in. Also, I was often able to kill boss before round 3, so his extra steps help. Jarlaxe with his range attack doesn’t help buff team AC, but can more often hit boss because of range and has some skills which can kill boss mirrors. You could maybe use other restore type hero’s too
Have a few options to get rid of boss mirrors. Row attacks, certain abilities, etc. I used ranger and warlock and Jarlaxe. Others could work. You need a few since it takes a few turns for ability to refresh.
I used cleric with healing boots and heal all allies ability. Between his heal ability and his healing boots, you can usually keep team alive enough. Rely upon healing boots, use ability heal in emergencies. If you can’t get enough heal out of that, you may have to include a second heal hero like bard.
I pretty much could have done it all with 1 key using this team. Just took breaks to open boxes and sell gear and stuff. My team is half level 19s with some 18s and 17s. So the lower level a team is, the more heal you may need and Tommus with AC buff sword would help
Hope the advice helps.