Any way to level up starter and partner past a point of no return?

You can suicide run dungeons in the main story to level your characters up. Just store all your items and go through the dungeon as you normally would, fighting things for exp, but don't complete the dungeon and just let yourself get KO'd on purpose to wind up back at the start.
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Repeat this as many times as you want to gain experience. You won't gain items however, but if you need to grind for items, there's pelliper island for that.
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Also, the walkthrough has high standards for levels, My first playthrough I was 29/28 for the final boss. Some have done it in low 20s as well.
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Be careful though, you can't gain Exp in the final area in the game. So make sure that you are all set as far as levels go.
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These are the dungeons I have unlocked. I would appreciate any tips to which dungeons to go through for gold bars, revive seeds, oran berries, whatever is most useful for getting through chapters 20-22(if these things appear at different rates in different dungeons):
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Water Continent:
Open Pass (Up to B2F) | Lush Forest (Up to 4F) | Foreboding Forest (Up to B4F)
Drilbur Coal Mine (B7F) | School Forest (8F) | Glittering Mountain (9F)
Nectar Meadow (9F) | Poliwrath River (B9F) | Ancient Barrow (B11F)
Sheer Mountain Range (13F) | Gentle Slope Cave (B8F) | Showdown Mountain (11F)
Revelation Mountain (13F) | Abyssal Badlands (B7F) | Cave of the Deep (B7F)
Calm Craggy Area (B7F) | Reverse Mountain (9F) | Cave of Training (Up to 5F)
Leafy Highlands (Up to 3F) | Sunbathed Gardens (Up to 5F) | Serpentine Mountain Trail (Up to 7F)
Uprise Range (Up to 3F) | Stone Field (Up to 4F) | Odd Field (Up to 5F)
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Air Continent:
Submerged Cave (Up to B2F) | Mystical Forest (9F) | Berry Forest (Up to 3F)
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Grass Continent:
Sunny Spot Hill (Up to 6F)
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Beyond the sea:
Fire Island Volcano (10F)
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Pelipper Island:
Pelipper Dungeon: Start | Pelipper Dungeon: Middle | Pelipper Dungeon: End
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I thought I had more on Air Continent, but I guess not. I never bought passes for the Mist or Sand continents.