Hello there Graum!
I guess you mean the Eden quest right?
So you´ll have to use the skills and collect the sparks when he is in "Rage" mode. I think you need 10 to be able to do the next step. One step will be to just attack with the spell you´ll have and the other will be dodging. After you did all of those steps it will finally die. It´s a bit tricky and maybe a bit confusing

but after you figured it out it´s easy

I just didn´t do it for a while so I`m not 100% sure about the correct order. If you´re still struggling I can try to see if a Reinc of mine has the quest open and do it again to see exactly what to do just let me know!
Hope that helped!
Greetings