Wee Witch can do true damage, I just assumed one would be using her in a lockup team and would never actually get round to it. Without a lockup, sure she can, but not a huge amount of it.
The Healer actually healing isn't what would be the issue, the issue would be his physical damage nerf which can go over 100% at higher levels and persists after he dies, rendering all minions in wave 2 immune to physical damage from skills. People complained about the same 'bug' in the King Imp boss battle - there's a stage there with red imps & a level 450 Healer.
Though if it wasn't a Healer titan, I'm not sure.