The monster is located at the bottom of the basement that Sasheer sends you in to retrieve the Leather Book, in Teusa.
The monster is accompanied by 2 Amoebas and the monster hit Hoston for 150 damage. There are probably many ways to beat the monster, but this is what I did and won in 2 rounds:
Have everyone gather 1st round. 2nd round, everyone send their gathers to Rudy and have him do Toxic Glare. It one shot everything.