Ghust

This boss can be found in the Lukki Lair orb room. It is an acid based slime monster which can leak massive amounts of acid and can shoot a large acid ball.

When killed, this boss drops a toxic cape upgrade which works just the same as Toxic Immunity perk.

Touch-Of Spells can be used to easily kill this boss as it is not immune to their instant-kill effect, however, you may need piercing to hit his center point due to the boss's size.