Groal the Great

Groal the Great is the gatekeeper boss at the top of Bilewater, the swampy Act 2 region (Bilewater is a place, not a boss in its own right). The bloated creature spits arcs of acid orbs, dives below the surface to resurface beneath you, and uses a vacuum suck to drag you in, while summoning a spike ball and firing Soul projectiles. Defeating it opens the way to Bilehaven and onward into Act 3. Staying mobile and punishing its surfacing animation is the cleanest line through the fight.