Grinding Pokemon Levels

Secret Base
Cool Trainer Trainer is the infinite battle partner in your secret base and can be a useful way to train and do battle practicing. Trainer uses Audino (6x) that has an EV yield of 2 HP. Take note that the trainer will only use 3 Pokemon in the level 100 selection,  so selecting the level 90 option is more beneficial for HP EV yield and experience gain.

Grinding After the Elite Four
The increase of the level of the other trainer's Pokémon seems to increase almost exponentially after defeating the Elite Four.

It is for this reason that the player will almost inevitably have to "grind" by battling high-level Pokémon to raise their levels. It is recommended that you have the Pokémon you want to train hold a lucky egg.

Leveling Up Water Pokemon:
Battling trainers and wild Pokémon at the base of Mount Nova

Leveling Up Fire Pokemon:
Go to the very top of Mount Nova, most Pokémon there are ice type

Leveling Up Steel Pokemon:
Go to the very top of Mount Nova, most Pokémon there are ice type.

Leveling Up Flying Pokemon:
Timelocked Temple (after defeating Wilson at his Hall of Champions) is full of Volcarona's, who are very easy to take down with flying type moves. But take a couple burn heals because its flame body will burn you often.

Leveling Up Ghost Pokemon:
Midnight Mansion. (After Defeating Odin/Zeus) the midnight Mansion is Accessible. The player can Either catch Darkai(Zeta Exclusive) or Cresselia (Omicron Exclusive) and battle inside the mansion.

Leveling Up Rock Pokemon:
Timelocked Temple (after defeating Wilson at his Hall of Champions) is full of Volcaronas, who are very easy to take down with rock type moves (4x effective). But take a couple burn heals because its flame body will burn you often.