Whether in the lava or not the elemental re-heals every turn. Can't hold, as not humanoid, and unless we can kill it in one turn there seems to be no way to defeat it. Any ideas?
Magic missile spam can bring it down. You could also try to lure it on land and throw a water bottle / cast the water spell, perhaps it stops the regeneration?
On a side note, I don't think it's actually hostile. It just walks around there.
