Ok.
In the payload phase, have someone from your team run to the end of the destination, that should prohibit anything from spawning. So there's that. Maybe they patched it tho?
After that, move your team and kill the first bastion going to the right.
Then have your tracer chase the second bastion, you can actually shoot him without him shooting you by hiding behind a wall and barely seeing his weak point on his back.
Then pray your team doesn't die to a random tank bastion. And deflecting orisa's bullets to her head does insane amounts of damage.
In the random heroes, pick gengu, he can do the same thing to the bastions as tracer, but his shurikens will do 180 damage. It's insane. You can even corner peak the bastion with your ult, but it actually does less damage that the shurikens.
You can also deflect 2 shots from the tank form.
I'll add to this, I did it as Tracer with a group of randoms, we did group up though. We had over a minute left with the following strats:
Do the streets glitch, normally this isn't the issue but it makes live a little easier. When in the plant kill all the turret Bastions first. Do NO kill the first Orissa as killing her spawns the other Orissas. When a tank Bastion spawns fallback with your team as they WILL tear you apart otherwise, have Reinhardt pin him for stun and damage. Having a good Reinhardt is mandatory as his charges do a major part of the damage and stuns. Remember that stationary Bastions when dislocated walk back to their original positions. Always focus the Bastions as a team and move as a team as the Reins shield is a life saver. As for hero specific tips, for the powerplant phase:
Tracer: try to focus the minions and the specials from time to time as these can put out quite some flanking damage, basically crowd control, hit the Eradicators in the back so their shield breaks allowing Torb to do some damage. Farm pulse bombs for Bastions and Orissa's, nice thing, you can play peek-a-boo with the turret Bastions, just fire a couple of shots in his core on the back and they will focus on you, hide, repeat, this can take some heat of your team.
Torb: don't bother with the turret in the last phase as it usually dies too quick, aim for headshots as you are a major source of DPS, if people die molton core and res them but keep aware of the surroundings, preferably with Mercy healing you, also molton core if you are nearly dying, being out of positions is team suicide. Did I metion headshots?
Mercy: didn't play as her but heal, save res for Reinhardt, unless there are downed people in the open. Don't bother damage boosting, whip out your pistol and farm ult.
Reinhardt: play conservatively your shield protects your team, engage turret Bastions with a charge (preferably move them as far as possibly), hit some swings and if he goes back into turret mode put your shield up to protect your team and allow them to finish them. When a tank Bastion arrives back up and the team should follow you. If you block his shot close range they do some damage to the Bastion itself. Get a good charge on him and hammer him down. Repeat with Orissa's.
I might have gone a little in-depth but I found that knowing your role on Legendary is very important. Hope this helps!