The Gluttonous Behemoth is a boss encounter on level 1 floor dungeons.

So you got the Gluttonous Behemoth boss in the runescape dungeoneering skill and he starts to eat from the carcass when he's low on HP and fully restores his hitpoints.

Well what you need to do is place yourself in between Gluttonous Behemoth and the carcass. That way he cant start eating. Pretty simple Uh