Grapple hook on to it. Hold to Reel yourself in, continuously feeding it while staying grappled. Rarely will hit you as it spins trying to reach you. Grapple wont releas because as long as you dont hit it, it wont count as combat
Diplo's are super tanky. Everyone doesnt realize this is now the BEST bullet tank in the game. When heath is low, sprint out with no problem. Unlike the bronto, the Diplo is very fast. While passive taming, realize that first feeding you have about 15 minutes until next feeding. Then all quick after that.
If you want to pasively tame this guy, you can place yourself somewhere where he can't knock you away. He doesn't do any damage at all, so placing yourself with the back against a big wall would stop him from throwing you. All you need is a lot of patience and mejorberry
If you don't have a ghille suit, build one foundation and place 3 walls around you and leave an opening, get it to come at you and run to the box. He will stay there with you forever trying to push you in your box, then just walk forward and check if he needs feeding