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.
Don't bother knocking it out. Jist get your back against a stone and make the diplo come right above you. The gap between the first two feedings will be huge so be prepared to sit in front of your screen for a while
When taming to make it easier stand against a cliff or a good sized rock. I tamed one for my brother and it was a piece of cake with that technique. Also if you find that the place you find the diplodocus isn’t safe just get near it and lead it away. They won’t hurt you they just like to swat you around like they are playing a game of soccer.