Invisible Geonor

I was mapping in the iron citadel area with geonor as the boss and im playing wybern druid and got him down to like 5% with oil barrage and then he stopped taking damage so i devoured him thinking it wouldnt do anything but i would try. He died and dropped loot even though it was the first phase but then he came back invisible in face 2 form. He killed me and then i tried to fight him again and he is still invisible and now doesnt even have a hit box. Failed my map because of it which is really freaking annoying since i cant rety and this is the first of the 3 of them that ive found.