Maybe I'm being silly, but I think the most frustrating monsters for me were early on in the game when I got out of the monastery towards Trynton, I ran into either it was some caustic vines or some crimson poppies, don't recall exactly now. A group of 8 or so of the darn things, all shooting blinding spores. Low level party without very good resistances so everyone was getting blinded. Ugh, not fun. I have fought monsters that hit harder and can be more dangerous, but nowhere near as just plain old frustrating as these things were making me miss miss miss miss miss miss miss (and I know appreciate a monk's ability to fight blinded a lot
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