Easy VenomMyotismon
After you get Metalmamemon from the trade center raise him to level 27. Afterwards go to File Island and buy a toy plane for 8000 BIT and go catch a Bakemon in Web Domain and raise him to level 20. Then DNA digivolve them together with Metalmamemon selected first and you'll get a DemiDevimon that stops geowing at level 31.
More tricks, tips and cheats for this game are right here - Digimon World 2 cheats
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