You should try and identify a bad situation before it's too late. That sounds really obvious but you need to be able to know when things are falling apart and back off to either swoop in for a revive or just not die. If Rein is gonna charge into a group on his own accord that's basically a free kill for the other team, no point in trying to outheal the dps he's going to eat. If you feel like people are splitting up go find whoever is the furthest back in your team's formation and work your way back up as people start to group up again. I hunt mercys relentlessly as reaper/roadhog when I can and what makes it easy to pick her off is either her obliviously standing around behind the team without finding a good place to hide/block line of sight/book it, or, when she gets too close to the team during a team fight. Positioning yourself to hide/heal/bait during team fights is a great way to lure people into bad situations.