Fastest way to level up at an early level well without cheating
First you should wait till marowak (the evolution of cubone) dojo opens up. Then go into the dragon maze and bring in to strong pokemon. Set your partner(optional it 's a risk) and the other 2 Pokemon to go the other way that can be done in tactics. You can go in tactics if you press on team. Then they won't follow you and they will attack all the Pokemon they see also the dragon maze is the best because it should give you the most experience.
Remember this will work on low levels though if you want to get good experience you can send in strong Pokemon with you to do half of your work because the Pokemon they defeat you get that experience too.
We have more cheats and tips for this game here, Pokemon Mystery Dungeon: Explorers of Sky cheats
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