Or just D.VA overload tank rush since her mech can't be destroyed by conventional fire when overloading and deals 1,000 points of damage. All you have to do is burst rush it past the shield and into Bastion and he (and probably Rein) are dead. At the very least you'll drop Rein's shield.
If you don't want to waste your mech doing that, hit the E key and laugh as you stop all offensive fire from Bastion for several seconds, while your allies unload everything on Rein's shield and drop it quicker than a fumbled dead baby joke.
Plus, most maps have multiple ways around and are cleverly designed with several height differences. Widowmaker will
still completely wreck Bastion and Rein won't know how to deal with it.
Then there's the option of Mei's ultimate to freeze them solid since she can pop up a wall to get in close and drop it to lock them both up for her team to murder them.
And then there's Junkrat's grenade launcher, which only needs a corner or some other location he can lob grenades over for him to easily manipulate the environment into blowing up any camping fucko to bits and pieces.
Then there's Lucio's ultimate, which gives his entire team a massive health and speed boost so they can simply bum rush Bastion like the 100 meter dash at the Olympics.
Russian chick (forget her name)'s ultimate makes them sitting ducks for pretty much anyone to fucking wreck em'.
Hanzo's ultimate doesn't give a single half of a fuck about Rein's shield and will rip through Bastion like a hot knife through butter. Plus his scattering arrow shot (if used properly) can kill Bastion behind Rein's shield if Bastion is up against a wall like he should be.
Bastion is probably one of the easiest characters to counter actually, because he's as one dimensional as it gets.