Video showing the strategy:

This is a consistent strategy for Gir-Tab in Axiom Verge. Git-Tab can seem a bit random when you fight him but he actually responds to what you are doing.

The basic idea with this strategy is to wait for him to sit down, then jump over to his back leg, fire once, wait a moment, then spam until he is about to finish his flamethrower. Then you wait until Gir-Tab sits down again. If done correctly, you should only have to take two hits and be able to defeat Gir-Tab every run.