Which character is best for killing Bastion from behind Reinhardt?

Bastion fires 35 bullets per second which deal 15 HP damage per each in a close combat, which translates into 525 damage per second. This means Bastion alone will melt the shield in approx. 3.8 seconds at an ideal situation. (My information may not be accurate, however. If that is the case, then just modify the numbers written here.)

However, the real situation is not that ideal. On our plus side, Bastion has damage dropoff which degrades their absurdly high damage output at a range, but the bad news is that Bastion probably has friends around them. I'll assume one standard hero such as McCree or Soldier: 76 would be around them, who has less damage output than Bastion but a significant threat against that gargantuan shield, anyway.

Thus, assuming that Reinhardt's shield lowers after three seconds, these are the heroes who satisfy the DPS bar:

• Pharah. Her rocket deals 120 damage at a direct hit and fires slightly faster than 1 round per second. Passes the damage check by taking less than 2 seconds (from the first rocket fire) for destroying Bastion.
• Bastion themself. Even without math, it is obvious that taking down an enemy Bastion would take less time than breaking Reinhardt's shield. Do note, however, that the offending Bastion might require a delay (which is one second) before stapling themself into the Sentry mode. They could consider installing themself onto a moving object beforehand (e. g. the Payload), or perform the mode switch while falling from a ledge to minimize the effective delay.
• Junkrat, if he can directly hit the target. Same damage as Pharah, with slightly faster fire rate. Note that, however, using him exclusively for this scenario is not an optimal strategy because of his trajectory.
• Hanzo. Each fully charged arrow deals 125 damage, and takes slightly more than 1 second to fully charge and fire. Thus, surprisingly, Hanzo can also pass the damage check when the enemy counterattack is thin aside from Bastion's.

Bonus point for Mercy amping anyone mentioned here, or Zenyatta attaching an Orb of Discord.

Special cases worth a remark:

• Roadhog actually has no means of getting through the DPS bar via shots. However, a test from the training room revealed that he can Chain Hook an enemy as far away as 20 metres, which means he can pull back Bastion safely if they are close enough. Although Hooking alone has minimal impact on Bastion's health, it forces them out of Configuration: Sentry, and pulls them into the close quarter where Reinhardt and Roadhog can shred them with maximum efficiency. Worth noting if your team is close enough.

Pharah can kill a Bastion with three direct rockets at any range (doesn't even need the Reinhardt if she peeks around a corner!), Widowmaker can do the same with three fully charged shots, and Zenyatta can kill a Bastion in 5 normal shots if he uses his Orb of Discord beforehand.

These are probably the three fastest kills, and they work at any range thanks to their lack of damage fall off, and their lack of spread and projectile drop.