Frosthand mercenaries - OP as enemies?

There's something off about Frosthand mercenaries, but i can't figure out what.

I'm playing RF chieftain with 8k hp, 90% resist all, 75% all block, 2.5k life regen, etc etc (basically endgame pohx), and i can wade through basically anything that isn't an uber boss without worrying about even getting scratched.

With *one* exception: Frosthand mercenaries.

For some reason or another, they completely melt me. They're the only merc i have to ignore (for now) when i meet them.

Can someone explain to me what exactly they're doing that basically ignores all my defense layers which literally no other mob, boss or merc does?

Frostbomb is turning off your regen and you kill yourself