Monster Crown (EP)

Directly right of the first town, near the group of mobsters, is a boss on a bridge. It's extremely strong defensively but if you apply infection on it then the fight basically is won in roughly 5 turns. Defending with about 3 monsters is enough for it's health to drain from infection.

Seycret and Laz both can cause infect.

Save beforehand and be sure to catch it. This monster is very strong early in the game and can heal itself. You don't want to miss catching it.

Note: Achievement is bugged in your favor. I got it after the monster defeated me in my first attempt.