Dev Blog: Regarding Weapons that deal a % of an enemy's health

[VROOMIGoRealFast]

Hmm so does this mean that future bosses will have lower health?

The short answer is no, but the Mausoleum isn't a good example of future boss health. The Mausoleum was designed to be a never-ending endgame zone, where the monsters further into the dungeon are always a higher level than you are. What this turns into effectively is that you're only dealing about half the damage you're supposed to be doing in the Mausoleum, which is then reduced by armor etc.

With the next expansion, we will not be doing the same Mausoleum trend (Mausoleum will remain the hardest content due to the level scaling), and the monsters you fight will be at your level, so you'll get your full damage. If the boss fights are taking too long due to players being too weak or bosses being too strong then we will adjust those values to make sure you have an enjoyable boss fight