enoch326

You are correct that he is killing you with explosive runes that he casts at you on the floor where you are standing at that moment. When he first appears at full health he will attack "normally". But, when he has been depleted of health to a certain level he will teleport to the edge of the room and become invincible while he casts those runes at your feet. When that happens, don't try to attack him, just run around the room in a circle to avoid the runes. In a few moments, he will go back to normal and you can continue to attack him as usual. This "invincible-rune" mode will happen a couple more times. I didn't check, but it seems something like when he is at 75%, 50% and 25% health.