Hidden Merman Magician at Ymir Cave

What this problem is related to? (e.g. Broken model or texture / Incorrectly placed model / Particle Effect / Animation / Collision problem / etc. ): Monster Collision

Location/Coordinates (/pos, if relevant):

Describe the problem: There is a hidden merman magician probably underground on this red stroked area, right near the edge of the water. Been killed twice already by this master illusionist magician with Improved Invisibility.

Just to note, I’ve been here before and sometimes I got attacked by him and sometimes not. Maybe its a respawn thing or a certain location he walks by and becomes invisible.

Yeah cave map is buggy, mobs just sink under map.