Overwatch's Bastion brings true meaning to the term "defensive hero" with his ability to set up as a deadly sentry and shut down whole areas of the map. He's also a bit of a delicate flower, but with a strong understanding of each map, Bastion can set singlehandedly ruin even the strongest pushes. If you like camping, you like Bastion.
Configuration: Recon/Configuration: Sentry [LMB] - Bastion's main source of damage is highly dependant on what configuration he is in. While in recon mode, he is able to move around and use his machine gun for decent damage output at medium range, but all Recon is really good for is getting into a good position to switch to Sentry. In Configuration: Sentry, Bastion becomes a merchant of death with a chaingun that has a considerable ammo pool, fast rate of fire, and amazing damage.
Self-Repair [E] - Bastion's Self-Repair makes him fantastic at going off on his own and finding good areas to set up in Configuration: Sentry without having to worry about needing a support to keep him topped up. Whenever you've taken a spot of damage, use Self-Repair. There's no limit to how often it can be used, but you'll be unable to attack or move so make sure there's a lull in combat before healing up.
Reconfigure [LSHIFT] - As mentioned above, pressing LSHIFT is what will make Bastion swap between his mobile Configuration: Recon and his static but deadly Configuration: Sentry.
Configuration: Tank [Q] - Bastion's third configuration transforms him into a cute tank that can zip around the map and fire explosive shots that do substantial splash damage. When the enemy team hears you activate your ultimate, the smart ones are going to run for the hills. In this mode, Bastion's imposing stature is condensed into a squat little tank and his good mobility makes it exceptionally easy to rush enemy formations and unleash hell.
Bastion Strategy Tips
Overwatch isn't even out yet and Bastion already has a reputation for being one of the most deadly heroes on the field. His ability to set up as a sentry with a powerful chaingun can stall the enemy team, forcing them to change their strategy and making you priority numero uno. Because of this, playing Bastion can be quite tricky. When you manage to surprise the enemy, you can feel like a god, but when the team is aware of you they're going to be working to kill you constantly. Because of this, I highly recommend swapping in and out of Bastion during the course of a match—as once a team adapts to his presence he only becomes a burden.
Using him effectively requires a good knowledge of sightlines and layouts of every map, as setting Bastion up in the open is a surefire way to get killed off quick. Instead, look for nooks and crannies or hard to attack angles covering objective points to make hunting Bastion as hard as possible. If you can, coordinate with your team to make sure that Bastion also has a good deal of backup should he need it during crucial moments of a match. Having a Reinhardt shield you at any time is a combo that'll drive the other team crazy.
Because Bastion is such a defensive-minded hero, playing him on maps where your team is attacking can be really hard. His Configuration: Recon does modest damage, but without more utility Bastion is wasted potential unless he can find a safe place to deploy—something you aren't likely to find on maps where you need to be constantly pushing forward.
When using Configuration: Tank, time the ability with a push from your own team. The damage Bastion puts out in this mode can be devastating, but Bastion still remains an easy target for any team that can keep their cool when they know he's coming. If the enemy team has any hero capable of burst damage like Roadhog or a steady-handed Widowmaker, they can shut you down before you even fire a shot.