I would highly recommend having at least 3 people for this although me and my friend almost did it with just the 2 of us at level 53 and 54. Haderax has a critical spot behind his head that you'll want to shoot in order to deal sustainable damage against him. You can use the geysers he jumps in and out of to launch yourself onto the side rocks to get a good aim on the back of his head, hence why I recommend 3 people so someone is posted at each spawn point. After dealing sufficient damage, Haderax will get staggered for a few seconds where you can jump down and shoot a critical spot in his mouth. You should be able to wipe him in a single stagger period if everyone focuses on the mouth but you have to kill him quick because if you take too long he will regain full health. I hope this helps anyone who may be stuck on the boss and not know how to do it.