Just keep an eye on him and look towards the direction of his dumb loud footsteps, anyone that can shoot him from mid range can shut him down.
A good Reaper always disengages at anything mid range. If you can hear his foot steps, he's already too close. I think the best way to deal is to either just be a better player than the Reaper with a close range character, set traps, use discord orbs, McCree or just lucky head shots with Hanzo lol.