Well, below is one example of why I think OwlCat's encounters design is bonkers.
Towards the end of act 1 there is one boss.
Then you can you can fight this hidden mob which is level20
And then you can fight another hidden mob.
Maugla (lvl18) >>>> Demodand (lvl20) >>>> Nulkineth (lvl??)
I'm glad that I won the fights after several tries but I don't understand the rationale behind this encounter design. The boss encounter is ok but the Demodand/Maugla are simply rigged fights.
Edit: It looks like these optional fights are part of the Sadistic achievement. Fml.