PTD: Pokemon Experience Points
Aside from learning new skills, a Pokemon gains status increases by leveling up - their HP, damage and other stats will rise and make them more effective against other Pokemon. This is why having a level 90 Fearow is better than having a level 50 Spearow. Here are a few tips for getting your Pokemon to increase levels as fast as possible.
Pokemon earn experience points when an enemy they attack dies. The exp is then split evenly among the towers that attacked the target. This means that if three towers attack and kill an enemy Pokemon they equally share the exp gain. To earn the most exp, place the Pokemon you want to prioritize in leveling at the closest possible position to the enemy spawn point. This ensures that regardless of who gets to kill the enemy, the primary Pokemon still gets a share of the exp.
To make this strategy more effective, it is important that your main Pokemon is able to attack as many of the enemies as possible (as opposed to killing them). One good strategy is to use an attack that hits multiple targets; try the ice beam attack for starters. If you are in a corner of a path - like in chapter 4, route 8, an attack with a radial area of effect is pretty effective. Status effect abilities also apply so try an ability that slows enemies down and prioritize "fastest" targets first. This will allow your Pokemon to target one enemy after another.
If the main Pokemon you are leveling is relatively weak (like a level 1 gained from a trade or a low-tier wild Pokemon), it is likely that they would not have any powerful attacks that are able to kill enemies easily. The next Pokemon in the line should have a really powerful attack - possibly one that is able to kill enemies instantly. This is why the next Pokemon should be your most powerful one. The logic here is that this will ensure that the experience points split is reduced as much as possible.