When taming, veggies wont work, so stand under its stomach and feed it mejoberries. When you stand under its belly, it will have trouble getting to you under there, so you don't have to worry about beong pushed off a cliff or into the ocean.
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.