A very easy way to level up monster character is to get any humanoid character to around level 400 then get to the cave of ordeals 4. There are 8 puppets there. Start with the enemies at weakest and kill them with your humanoid character until they level up. Keep doing this until you feel comfortable enough to raise the enemy difficulty. Continue this process until the enemies are at strongest. By this time each of them is level 1080. When you feel comfortable enough that your humanoid can kill them easily magichange any monster with them then kill one of the puppets. If the monster is level 1 it will be around level 250 after killing about 6. And it will have about 50k mana. Just repeat this process until you are satisfied. I did this and all my monsters are level 1000+ and my humanoid was level 4000 hope this helps ^^ keep on leveling.
Note: to unlock magichange you need to beat Axel mode which is 4 chapters long. Its best to start in Axel mode so that it is much easier.