How do I run just a single stage in my bamboo build?

From the Atlassian help forum, here: https://answers.atlassian.com/questions/52863/how-do-i-run-just-a-single-stage-of-a-build

Short answer: no. If you want to run a stage, all prior stages have to finish successfully, sorry.

What you could do is to use the Quarantine functionality, but that involves re-running the failed job (in yet-unreleased Bamboo 4.1, you may have to press "Show more" on the build result screen to see the re-run button).

Another thing that could be helpful in such situation (but not for OP) is disabling jobs.