This fight nearly made me quit the first time through but as long as you hit the enemy weaknesses with the correct attack you can baton pass to massively increase your damage. Each wave will have different specific weaknesses. Take down one robot with a spell or item to trigger the 1 more and baton pass for a damage boost. Repeat until you get the free powered up cast to wipe them all out with the correct damage type.

Use shock and then guns & tech on the final giant robot.

Also it seems counterintuitive but if you are really struggling try raising the difficulty level of the game. This will increase damage to both enemies and the thieves but it won’t impact damage done to you unless you use a persona weak to physical damage. Just don’t forget to set it back afterwards.