Symmetra. Spam orbs at Bastion.
Don’t get fooled, you will NOT kill a pocketed bastion with those orbs. If they stand still and keep shooting, your orb can’t outpace Mercy healing alone, much less Moira’s. But the objective here is not to kill Bastion. Is to kill everyone else grouping with him.
Both tanks in front of him, and both healers behind him need to move out of the way of the orb. Moira can’t sustain her group healing indefinitely, and if she gets distracted and bite two orbs, she is dead. Usually someone dying behind shields (especially a healer) make the enemy team panic very often in gold. If you are lucky, even your orbs can make Bastion panic (despite he being totally safe being pocketed) and move away, and your team can use the window of opportunity to push.
On top of that, if you need to cross a short distance to reach Bastion, Symmetra barrier is surprisingly resilient, and even Bastion will take some time to mow it down. That is two tools she can use to get into a President Bastion formation.
Second option is using Mei, and just creating a wall directly under Bastion, raising him above the shields. This will mess his aim for some seconds until he notices what happened, and your team pretty much have a prize target to shoot at. If Bastion switch to Recon and move, break the wall as soon as he drops, and push together.