Train them each individually on level 1 of the forest. Bring potions and buy moves. You will eventually make the money spent on these back. Try to kill rabbits and squirrels to start, but as you gain moves and increase their battle and resolve stat, start killing foxes. I have found that badgers are harder to kill and do more damage so fight them when you think you're able to.

Make goals for yourself. I like to try and get at least 100 kills a day, but if you're just starting out shoot for 25 or 50