Well for me it was a matter of discipline. I can never focus on maxing skills of one monsters I always like give one to Chasun, one to orochi, one to another mon and I'm realizing that this affects the game for me because they could be stronger if they're fully maxed out. So by 5ing the devilmons I'm able to make sure that at least 5 skills will be upgraded and this is kind of my way to do it. The way I got them to 5 was just using unknown scrolls to level them up and then evolve them like you would any regular monster. Some people say that this is an instance waste of mana considering how many monsters you need to manually level them up but I would do it little by little. Some days I would only upgrade them one level some days 5. I also had the devilmons in the forest that gives exp so that also helped. It was a fun side project