[@prettydrops][@TheForgottenArc]
I was thinking that leveling up could simply be done by any sort of "training". This includes battling wilds, training against your own Pokemon, or even excersizing or running solo drills. Basically, any post where a Pokemon is getting experience, you can then level up the Pokemon. (Again, I'm going to let y'all keep track of your own leveling up.)

As far as slowing down leveling up, I think I'll just say this:
While a Pokemon is below level 20, they can gain up to 4 levels in one day. At level 40, they can gain up to three levels per day. At level 60, they can gain up to two levels per day. And once they hit level 80, they can only gain one level per day. 

Sound fair?