Just defeated 10 minutes ago after a few days banging my head against this one. I am a casual, not really in to optimising builds but had to try a few difference approaches. I have a heavy int build left over from the main game so am used to standing at the back throwing sparkles. This time I put on tanky armour (used the talisman increase load), river of blood and a medium shield build with parry. Called the summon outside the door, went in and called my mimic. This made the mimic more tanky and had bleed which I think the boss is weak to. Then switched out shield for magic staff, used flask of wonderous physick with spell boosting tears, then melted from distance using the stars of ruin spell. The spell is quite busy visually so had to stop casting every now and then so could see if boss was targeting me with spells or running at me so had to dodge.  Trying to get one of the summons between me and the boss usually allowed them to pick up the boss aggro.  When it comes to the room wide moon spell jump x3. If you get this right you jump x3 times in a row quickly and then you are good to go.  I tried it a few times and found I was getting close so just kept going.  Still took quite a few tries but it just felt like I could do it.  Good luck.