Aim for their crotch. That may seem wierd, but with the way their hitboxes are, you actually have more area to work with in their lower bodies than upper body.
I’m talking of course in the 2.5 meter and beyond range.
Which feels odd because your reticle kind of aims at the floor since Reaper is much taller than them, but the math works out. Since their bodies are roughly triangle shaped it makes more sense to aim lower to increase the chances of Reaper’s RNG working.
Before I was aiming at the middle of their body, between their stomachs and chests, but a surprising amount of pellets manage to go above them.
For most other 200 hp characters aiming in the middle of their body works because they are more rectangle shaped: Soldier 76, McCree, Mercy and Widowmaker especially.
A note on Widowmaker, her upper body is rather skinny compared to the rest, so unless you are so close her head us larger than the standard reticle I would shoot at her chest at the most. Aiming at her head even within the 2m mark gives inconsistent results during fast paced movement.
A note on Mercy, she stands with her shoulders at an angle relative to where she is aiming. This means that while her character model is more or less just a slightly smaller Soldier 76 or McCree hitbox, it’s actually being made to show less surface area from the front since she’s twisting her upper body while holding her staff.
So, if you are up close to Mercy (between the the 1.5 and 3m mark), but she’s standing with her shoulders twisted parallel to your point of view then aim for her breasts. Again, that’s not a joke, you have to compensate your aim for the hitbox of the individual character and their stance which changes the surface area of their hitbox.
Genji is really odd because he’s really crouched, but still roundly ball shaped. I haven’t found a consistent way to kill him yet with Reaper. Right now I use the melee, shoot melee combo on him. I’ll get back to you guys on him.
A final note:
Be wary of practicing Reaper on the training range. The robots have an inflated hit box, and are more or less triangle shaped (with their huge heads being the base). Which will train you to aim high for critical shots. The only character shaped like that is Reaper because he has a body hit box like McCree, but his head hitbox is rather large. So the bots will train you to deal with other Reaper’s, but not the general cast.