OMG GET A TEAM THAT HAS AT LEAST ONE PERSON THAT TALKS!
This is hard as nails, seriously-here's how I did it:
Reinhardt - Me. Focus on shielding ALL THE TIME. For context, I had 24 elims all game, less than 2 others. Only times you shouldn't shield are 1)when you are overwhelmed, use charge to reposition everyone-or 2) shatter at the heavy -or 3) when your shield is dead. Your hammer is NEAR USELESS, so if you are gonna strike people, aim down to hit less people. USE YOU MIC, ORGANISE YOUR TEAM. Keep your team updated on your shield, attacking enforcers, and snipers
*NOTE* One tip I got in my second success(in a row, this works) is using charge to throw assassins off others is a clever idea IF YOU ARE NEAR A WALL. Otherwise, get the others to punch him/her
Bastion - You are PROBABLY THE ONLY GUY HARMING STUFF! PICK PRIORITIES! Here's the priority bracket:
-Sniper/Assassin/Heavy - Enforcer - Trooper
Shoot everything. Then shoot some more. And shoot more. And then some. Seriously, don't stop shooting
Mercy - Easy enough. Pocket bastion. Had zero elims both of my runs, only heal others if they need it. Rez as fast as possible
Ana - Use your anti-nade to remove the heal aspect from groups, you can DPS, !IF! the bastion is shooting them or they're anti-ed
SLEEP THE HEAVY! IT WORKS. For a little bit use this to retreat/revive
As you go, stay together, be slow and careful. There's no timer until the final bit, where you GET ON THE SHIP!!!!!!!!!!!
FIRST HEAVY-
Go about this one of 2 ways. Either, kill the heavy first or everything else. Either way, rein can shatter to stop the heavy, same with ana sleep
Back up as far as you need, there is a large amount of space
SECOND HEAVY-
STAY ON THE ROOF! When the heavy starts shooting, use rein to draw fire, shield and ana/mercy spam to keep him alive. Let bastion use turret to eat him alive, only use rein's hammer IF the enemies are anti-ed, we almost died here 1st time because there were enforcers around
Finally, at the end-DO NOT MESS AROUND. If someone die, tell them to f*** off and get on the ship. Or, you could take on 2 heavies