You can melee the assassin when she jumps on an ally to knock her off. I see so many people sit there shooting at her or Moiras trying to outheal the damage (you can’t) instead of saving their ally from their impending death.
You can also stop her with McCree’s stun, and dashing away from her with Genji will also stun her. If Reaper goes into Wraith she’ll just stand there for a few seconds (not sure if the same applies for Moira’s Fade but I’m guessing it does). These all give you opportunities to get off free shots on her.
Thanks for reading and happy killing