Go out right spawn door and go through the building on the right side. Position the dive from that point.

All in with all of D.Va's DM and Winston/Hammond and your DPS. Trades are fine. All you have to do is kill the bastion and prevent the res and you'll probably win the first point.

Other option is to play Hog/Hanzo/Junkrat(this doesn't work as well if they have rein). Slow burn the first orisa shield, once she drops a new one, use storm arrow + junk to burn the new shield of cooldown, then hook the bastion down.

Don't worry about managing the fights. When someone runs a bastion bunker you just have to win one fight against them and then it becomes really easy to snowball.