There appear to be a handful of healing mechanics that Psyker MoBs use to heal themselves and surrounding MoBs. The ground effect (purple-hued mini-zones) seems straight-forward enough and can be avoided as necessary. The brute MoB has an shout (yellow skull buff icon over the NPCs when active) that converts damage to heals, so that can be avoided.

However, there are some villains (the Baleful Champion is the one I recall offhand, but there are a few champions that instigate this mechanic) that self-heal to the point that my character cannot make a dent. I can get them down to 80% heal, but they immediately self-heal to max, countering any damage my inquisitor does. I have tried to watch for an effect on both the villain or my character, but I do not see a noticeable effect. I have tried re-positioning the combat, avoiding doing damage (in case it was an inverse effect) and several other tactics, but, I am coming up empty. What am I missing/overlooking?

If it matters, my WoC is the heavy flamethrower, but I don't think its as simple as damage type or straight damage mitigation.