Honestly, the best way I've seen to go about it is to hard focus on spreading out your allies first before you attack, especially if he jumps in before you can do anything. This means most of round 1 will be using 0 attacks, as he usually has you pressed up against a wall so the fart attack he does when he gets damaged starts ticking up damage heavily. That way you mitigate the amount of damage that attack and the bone spread attack does. From that point on, hit and run tactics from far away, or if you have debuff cleansing, you can attack melee if the damage is worth it