This guide applies to all four of the characters, as they nearly all hit level 10 at the same time.
This achievement is very straightforward and you should have no trouble completing this, but to ensure you don't get any achievement glitches, it seems to be safest to do these four achievements AFTER pulling the plug (as pointed out by Stifler4Life in their solution to the Pirates of the Enchiridion achievement).
You should be at around level 9 on all characters after completing all side quests and the main story if you attack every enemy you see. After that, the Fire Kingdom is the best place to grind, quickly drive around the kingdom in a clockwise direction, leave the area so that it loads the overworld, then sail back in.
-Always attack enemies your level or maybe one higher, but attacking way higher is usually a fools errand.
-If enemies are friendly to you (such as cinnamon bun's guards or the banana guards by the admiral), simply hit x to make sure you can grab XP from killing them.
-I'm fairly certain you earn less XP for a character if they get knocked out, so make sure to revive them just in case.
-Always make sure the final blow is done by somebody not yet level 10. This can be done by making sure any level 10s do their defensive move while the last enemy's health is low.