Originally Posted by
Danko
Kaboom the Demolitions Master is immune to fire. It cannot be stripped: Tiefling's Ash+Improved Scorch does not remove the immunity. I suspect Kaboom's fire resistance is set to some crazy high value that simply overrides whatever Improved Scorch does to strip immunity.
This same fire-proof issue shows up in The Archon's Trial final boss Rhi'enne the Planetar. In the level 30 version of the quest, Sorcerer Fire Savant Elemental Apotheosis does not strip the fire immunity either. I suspect this will also apply to the Legendary version of Undermine.
These are the only two bosses in the game I'm aware of that behave this way with regards to fire immunity/stripping.