2 Frost Chromatic Orbs can do miracles (3 if you are willing and able to use metamagic) ...
You can also use legendary staff, if you feel like 1Dmg will help.
Also i usualy pulled it out of the lava.
Either by tadpole grip from Lae'zel, or by simply standing on those stairs ... he will come to you.
And if you have Wyll in your party, dont use Scorching Ray it will not give any damage, but it still proc Hex ... Eldrich Blast would be probably better tho.
And finaly ... this kind of question would better fit to this part of forum:
https://forums.larian.com/ubbthreads.php?ubb=postlist&Board=88&page=1 
I killed it on first or second turn every time ... the poor guy never stood a chance.
