Rare mob abilities that should not be stacked & Exp loss in the late game.

I think unfair combinations of rare mods is actually fine. It's ok sometimes meeting something insane. The problem is mostly the frequency and the nature of the modifiers (all modifiers that aren't raw stat buffs either force you into melee or punish melee, with circular effects that makes melee's defensive movement benefit - running around the target - worthless).

What I'd do is add more modifiers to the pool that impact movement and do something other than "kill you if you stand close to it"/"force you close to it". Grenades (that only explode after they have travelled a certain range, regardless of whether they'd "hit" something as normal projectiles do), walls (specifically countering ranged skills from one direction), cone attacks (ie a broadening line that can easily be avoided the closer you are to the monster), and generally make the directional attacks have less turn rate overall. Because rare monsters gain their modifiers from a random pool and there's a cap (mostly), this will dilute some of the more annoying modifiers (making it rarer) and diversifying the number of builds that can deal with it without just outgearing it.