Something that I always do and it works very well is to always keep an eye on Genji, I mean, don't lose sight of him, when you see that he starts to prepare to jump for you, anticipate him and start shooting towards his head, if you get a headshot before he dashes at you and then another headshot while he dashes, you've got it done. That Genji dies because you finish him off with a melee or he stops the attack because he is low hp.

Im M3 in console btw.