Fire lady first
Fire conference call
Slag conference call/pimpernel
Grog ( if you feel like you need it )
Leg soldier/Neutral Evil ranger
In the backpack:
Try to get in one of the safe spots shown by multiple users around there using the toothpick+retainer speed boost, then wait for him to spawn in front of where you are. Place your turret on a wall while he is underground, that saves time btw! Once he comes out, slag him. Once he is slagged you can do two things. You can shoot his crit spot with the lady first or shoot the sandhawk and then switch to the lady fist for the extra critical dmg. He should be stunned there. As soon as he begins his stun animation, jump down and try to slag him again, you can throw a slag nade, use the pimp, whatever, but slag him. Once you land quickly aim for his big mouth critspot with the fire conference call. I have farmed him at op10 with good consistency using this tactique, let me know if you have any fortune with it!
You need to be really precise and get the right timing with this guy. He is basically a DPS test. If you manage to produce a sufficent amount of DPS while he is stunned, you win, if not, he WILL heal, not matter what, sometimes he will not, but mostly he kinda does. So yeah, just get the hang of it and Haderax is a patterned battle