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? It is very useful when you can not afford misses. It's the offensive spell I have used the most.
A sidenote: this might change when they introduce the shield spell and reactions but overall, it will still be a safe bet against non-spellcasters.