>>57953357
Dude Gen 5 literally gives you catch up EXP no matter what you fight so you're always at the level they want you to be, just play the game.
Also in what game have the wild Pokemon ever matched the gym leader's levels? The gym leaders are always deliberately higher level than everything around them because they're supposed to be a significant increase in challenge.

To be honest I still haven't figured out XYZ summoning