Hulkbuster proves that for a genius, Tony Stark can be really stupid.
Okay, you've got a green rage monster with nigh infinite strength who just gets stronger the more angry he becomes. And your plan to battle him is to build a suit which will cause him pain and make him more angry..... "Not a great plan" to quote Tony Stark.
If they REALLY want to shut down the Hulk, when he's trapped in that little metal room at the start of the fight, you start playing a lullaby and spraying nitrous oxide in his face. Show him pictures of Scar-Jo in a bikini. Do things which will calm him down.
The weapons on the Hulkbuster suit should be designed not to cause pain, but to cause pleasure. Fire a laser which would stimulate the feel good centers of Hulk's brain. You need to calm that boy down!!! Turn him back into Bruce Banner. Fighting him is counter-productive. I can't believe neither Tony Stark nor Bruce Banner realized that when they were designing it.