Tips to Defeat
For most fights you can ignore targeted resistance builds. For Vaal, put three points into miasma resist. In fact, it’s a good idea to leave 3 “spare” deco slots on your build just to negate the current monster’s special skills.
Another +3 raw of Attack boost isn’t THAT important, having a fun time learning the fight is.
The miasma blight cuts your max HP by 50%, then you take damage, and overall it’s just too painful learning a fight while getting knocked down to 50HP all the time.
Get the immunity and the rest of the fight is easy, because he doesn’t really hit that hard.