The Titans will not appear until late in the game. There are enough Titans to get this if you do it on every one you see, but if you don't have it by the end of the game, load up the Tides Challenge and on wave 8 you will see one. They will fall to one knee after they get to half of their life. It is extremely awkward trying to jump on them, but it will eventually let you. Once you are on him the game takes over the movements, and you must hit
to perform the Aerial Slash before it's too late. Doing this without pressing the button fast enough will result in you falling to the floor. If you are worried about timing it, just keep pressing it until it happens after you are on top of him.