Unfortunately, as much as it pains me to say this, Bastion is viable as an offensive hero. Don't get me wrong: it is far more effective on the defensive team, but, as I've mentioned, I've seen Bastions on offensive teams more times than I care to admit. If it's an escort map, you can set up shop on top of the payload. But that would only work well if you have one or two shield tanks to sit there with you, plus flankers to deal with enemy Junkrats or Bastions before they shred through your protective teammates. Alternatively, you can be extra sneaky with your Sentry form positioning, as your enemies on defense will certainly not be expecting a Sentry Bastion to come to them.