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What level are they? Just find pet battles of similar levels and fight them.
A good way to do it is to follow the Master quest chain. Just level them up in the area near the master and it normally works out fine. If in doubt, the zones approximately follow the same scaling as what level you should be to quest there.
They are level 8. And I tried to follow the master quest chain but the master usually kicks my ass.
Then level a few different types of pets up that work well agaisnt the master you are getting stuck on, i always tried to keep 6 pets around the same level so i could switch them in and out depending on what type of fight i was haivng problems with.
Once you get your pets high enough also, you can take them to a higher level area and level your level 1's there really fast, for example, I have a team of 2 level 25's and the 3rd slot goes to a level 1 (the first slot) take them to a level 25 pet zone, start battle, let your level 1 make one move, then switch it out for your level 25's beat them, and you get that level 1 pet to level 3-4 rinse repeat, you can level them to a certain poitn pretty fast doing this. Would work with non level 25 pets too ofc.
As the other posted mentioned, stick to the zones that are whithin your pets range, I was battling level 14-15's with my level 11-12 pets pretty easily, some tougher than others. but the masters are very good to beat as you get amazing xp from them.