I don't know monsters very well because I am not fan of hunting but..my teammates always scream KILL FIRST INK BLOB (on the way to library bosses). I suppose Ink Blob is really strong monster.
There are two Biting Books located in Issavi. I died by two of them on my 600 MS.
Melee (0-1055+), Spit (0-1210+ physical damage), Bat wave (0-1210+ physical damage), Dust cloud (no effect on players)
In my opinion Freakish Lost Soul can hit us hard as well (maybe not alone but they are always in group).