Generally with Reaper you’re going to wait until he’s not with his team to go in.
You have to play around his teleport and be mindful of where he is.
Any time he uses his teleport to get away from his team or dive your back line, that’s your queue to go in.
Why? Because Reaper isn’t there to defend anyone.
Best case scenario, he uses a CD to dive someone, you use that opportunity to try and pick off a support.
Reaper will obviously follow, so when he’s caught up, you just dive back into your team.
Rinse and repeat and if you get a pick, it’s very successful.
If he’s sticking with his supports, that’s fine too because Reaper is useless at range so you’re forcing them to have 1 dps be useless while you create space with bubble and poke with right click.
Bastion is the same deal, you hide when he’s in turret mode, second he’s not, go in and try and pick off a support.
9 times out of 10 their team doesn’t have the communication or co-ordination to shut you down even with Bastion/Reaper.
For example, Reaper will get greedy and want to dive your team, or bastion will go for plays, their supports will position poorly and not respect you etc.
You can also use your bubble in more creative ways.
For example, you want to dive Zen, in front of Zen is reaper and the tank.
You could dive between Zen and Reaper > place your bubble > walk away from your bubble towards the Zen so now Reaper has to walk through your bubble to catch up to you and then > jump away and hopefully you picked off Zen.
TL;DR
Be mindful of their positions, make plays where they’re not and play off of their cooldowns (reaper teleport, bastion turret mode) and be more mindful about how and where you bubble
EDIT
videos like this can help too
h ttps://www.youtube.com/watch?v=h_SRuD0elDI&t=481s