When you grab the sword and jump on the right side rock, you crouch to activate invisibility. Your team should not shoot at Crota until you are about to go invisible. You need to be the one that tells them when to start shooting Crota. You need to tell them right as you are about to turn invisible. When you turn invisible they should already be shooting (or firing their first shot at) Crota. You should jump up there immediately and he should be falling to his knee right as you get to him. The key to all of this is timing. This is where consistency in your fireteam (or communication if you are doing it with new fireteam members) is key. What weapons they are using to shoot Crota makes a big difference in how fast he goes down. They should use the same fire pattern (same weapons) every time so you can get the timing down on your approach and anticipate the right moment so you get there when he falls to his knee. Good luck!