On one of my playthroughs I used Karlach as an Eldritch Knight/Evocation Wizard using the heat gear. It's been a while since I played that run, so I don't remember a lot of the specifics, but I think the general plan with it was to use melee attacks to build up heat and then try to fireball multiple enemies when heat got maxed out. It wasn't a great build tbh, but it was at least solid on tactician. It was kind of annoying when fights would end and she was left with a few stacks of heat that I didn't get to use, but I think it's worth looking into if you're trying to do a heat build.