Yeah... so my philosophy here (this is EE work, not CM Patch), was that campaign bosses shouldn't be unstoppable juggernauts. What should make them interesting is unique designs and a variety of special attacks you don't see elsewhere. Their stats aren't weakened in the mod but they're not strengthened either.
On the other hand, those out-of-the-way bosses, completely optional to progressing, that NPC's look at you in awe for beating... well those got some improvement. All four elemental lords in particular got standardized (they were all over the place in terms of stats) and boosted values.
You'll definitely want fire weapons vs. the Ice Lord. But your comment has me thinking, each one should probably have a more pronounced weakness. For example, it's true the Ice Lord has no fire resistance, but maybe he should actually have a fire vulnerability. Many enemy types do after all, it's like reverse damage mitigation that makes them more sensitive to a given damage type.
Something to add for the 2.4 release.