As the strange bubbling and gurgling noise continued, I knew what was happening, but I have nothing left to stop it.
I used up most of my remaining mana in that strike, and both my arms are useless. I could try and crush his body with my legs, but striking a person's body with his level of cloaking would do little more than tickle.
Sitting down and resting, it took a solid minute for the brain goop and shards of the skull to recollect at his body, eventually reforming his head.
After about two more minutes, the bastard was back in nearly flawless condition. Based on how long that took, it seems my theory of burnout was correct.
That, or destroying his head simply took a ton out of his regenerative capabilities. More information for the future never hurts, especially if I have to fight this battle-hungry monster again.