If you are using New Game+ to access Overlord route, just use whatever stages you can access, chip away at weaker enemies with the group. The first stage shouldn't be unreasonable, if you could beat the main story with Lv51 Prier.

If you are using New Game to start the Overlord route directly, be willing to go back to a previous chapters stages, or hunt down single enemies for a bit of exp until you get some skill levels.

Both routes into Overlord route expect you to use the stages provided to grind as needed for whatever is ahead, and NG+ Overlord route is scaled up to start around the same as the final chapter of either NG route.

While normal Prier is a physical powerhouse and a Warrior archetype (high HP/ATK/DEF, poor elsewhere), Overlord Prier is instead a Glass Cannon archetype, her durability drops in favor of more attack. This takes some working around, but once she hits her stride she rapidly starts to dominate. Overlord Prier really wants some levels on her personal specials, they get significantly better.

Reminder that most human allies (all but Homard IIRC?) can Purify standard monsters, as can anyone equipped with an equip that has the Purify special. With the equips, it applies when hitting with normal attacks, including when counter attacking. You can use this to get some extra allies, if your current squad is not cutting it.