For an easy tame bring them to shallow water then bring a taming boat and drive it right into them, stopping when they're legs are in the middle. If you do it right they'll now be in the boat and unable to run away and they'll be safer from predators. All you have to do after that is stare at their belly, occasionally throwing lystro kibble at it.
Know that it is easiest to passive tame them, which means walking up to them and feeding them
Get a TON of berries, everytime you feed it, it will go up like 3-5%
Have a high speed, high attack flying dino, (high attack in case you "miss click")
HAVE PATIENCE, diplo's are a stubborn tame, they will push you playfully (hence the flying dino)
MAKE SURE there are no carnivores around, if there are carnies around it will attack it there is a chance of it dying, it will get rid of all your hard work, and most importantly, IT MAY DIE (R.I.P. attempted tame #1 + 2 😢)
Plz note I am not an "high level player" that has a giga, I am lvl 50, (51 at the time I wrote this, almost 52) so I am not that experienced, tho I am sorta, I don't know that much about the game
I used this while taming my diplodocus, Get to a spot where the diplo is stuck and can’t hit you (so it keeps hitting and doesn’t eat other than you feeding it) and then rinse and repeat (be careful for carnivores)
The easiest spot to avoid being pushed away is in between it’s front legs. It will sit there and repeatedly do the pushing motion but you will remain untouched so you can tame it without running all around trying to get near it.