I'm thinking Transformers stomp. They are like 5+ times bigger than the Hulkbuster armor and don't have any humans within that can be battered and bruised.
If Veronica can repair the Hulkbuster that may help, but I'm thinking Optimus and Megatron turn it into scrap metal in no time at all unless the Hulkbuster flies off and fights from a very far distance.
Also, is Megatron still in the nanomachine form from Age of Extinction? That movie wasn't all that good and I haven't seen The Last Knight as it looked like hot garbage.