How would I go about practicing Genji ultimates? I have over 200 hours on him and still regularly screw them up.
If you don't dash as high, you can save time by unsheathing the sword while you're falling and save the dash for potentially intsakilling them with it by animation canceling, this is especially during nanoblade. I like to practice it on the part of the practice range with the moving bots. If you find supports grouped up, you may not even need to dash up, because you don't need to select a target. Dashing up does allow Anas to nano you more easily, as long as you warn them. (Warning them depends on the rank you're at. At the lower ranks, telling them exactly what you're gonna do is important. At higher ranks most Anas only need a "let's nanoblade")
Genji ult hitbox is very buggy, making hits more likely if you swing above enemies heads or jump and look down at them. You can also improve the hitbox by moving your aim in the direction that the blade is swinging. This is why people like ShaDowBurn's aim looks very shaky.
Remember, killing both supports is an easy way to win fights. If you kill both supports, then you can try to kill dps or low health tanks