There's really no restriction as to where to train.
I just went over to the Desert area and trained around that area (the Digimon's Level was roughly around 18~20).
But that's me, I fought lower levels mainly because I wanted to peace-keep my record, but you can go on to fight Level 25 Digimons if you want.
If you want to power-level though, make sure to stock up on Booster Chips. I level roughly every.. 20~25 minutes or so since I can one-hit everything with my latest F3 skills.