It'll be a combination of achieving the required level, and bond with their trainer. Once they've achieved the required level, we as players will be allowed to choose when they evolve.

For instance during mid-battle, when the need arises, other dramatic display. We don't have to play this exactly like a game after all.

Honestly I'm pulling up a lot of inspiration from the anime and how they seem to do things. I think it fits.

Also lastly [@AbysmalDemon], what Pokemon do the Nexus Grunts have? Also, I've posted. Take a look at the battle and tell me if anything's up.