A swarm of magical Hohen (forgot the actual name of the monster). I (if not everyone) considered a magic type monster is the most dangerous monster (except for World Bosses, of course) in Tree of Savior when the level difference is small. Even when you’re over-level-ed, the magic monsters still possess a certain degree of threat. Even just fighting 2 or 3 can kill some CON based character (where each CON status gives an increase on maximum HP for a character).
Now, my character is an INT based, and I’m screwed if I’m faced with this battalion of magical monsters. It’s time to use the warp scroll and get the hell out of here.