You play the game with YRP on Level 1/1/2 (can't avoid one of them reaching Level 2 but if you kill off the other two during the first Leblanc fight they should stay Level 1) and may only use the First Steps Garment Grid.
Now you might be wondering just how you're gonna go through the game without leveling up, well there's no real way to do that; if you use YRP they're gonna level up. There's the Minerva's Plate accessory which makes you gain no EXP but it completely skews your stats. Some enemies put you in Pointless status which prevents EXP/AP gain, but those are rare to come by and of course it's cured automatically once you return to the airship.
So what do you do? The easiest solution is to use the Pointless status whenever possible, but otherwise simply fighting bosses with YRP, reloading once you succeed and then doing it again with creatures instead of YRP. Like that you can fight all bosses with Level 1/1/2 YRP.