You don't have a standard counter to him but maybe Claire could work? She won't be able to avoid the damage but she can heal back up and power-drain him. She can also build up her charges by throwing incinerate attacks, since he's immune.It would probably be a really tricky fight since you'd have to cycle between her different modes a lot in order to keep him under control, but it may be your best option.But I don't know if I've ever tried using Claire against him so I can't say if it would work, and I don't remember the specific nodes for that fight in particular. Edit: Does anyone know if Killmonger's indestructible charges keep him from taking damage from plasma detonations? If so, he might be an option.
https://forums.playcontestofchampions.com/en/discussion/128890/6-1-6-havok-is-just-a-bit-sillyUnfortunately you don't have good counter atm.6* Havok duel: papa hunter