Negotiating with him is like playing checkers with a pigeon. He'll threaten to upset the board and kicks all the pieces to the ground, and everyone will wonder if he's gone mad and breathlessly report on how dangerous/unpredictable/crazy he is. That's not "unpredictability". That's easily predictable.
The only real trick to negotiating with him is appealing to his ego and/or making him think he thought of whatever solution himself. Or just straight manipulation: Play along at first, pretend to be incensed at his outrageous antics. Begrudgingly give him some meaningless "win" after a couple of rounds of negotiating that he can trumpet and he'll declare victory in the press. Do that, and he won't upset the checkers board. Predictable.
Mexico seems to be going this route. Eventually, they'll probably agree to increase their own border security and designate more federales there and will clamp down on the Chinese fentanyl being imported by the cartels or whatever in exchange for no new tariffs. It will make ZERO effective difference because they can't deliver, but Trump can claim he brought them to the table and got them to play ball and that now everything will be just peachy do to his brilliant and aggressive plays that nobody else would dare to do. It probably doesn't even matter if Mexico actually even makes any changes in the following months.
So yeah, pretty easy to play his game.