Train them, like a normal person would.
Not only will you train up your Pokemon, but you can work yourself towards completing your Pokedex by having encounters with Pokemon you don't have/haven't seen before.
The easiest way to level up is to feed Pokemon Rare candies.
you go to a Pokemon Rocks USA
the easiest way to do so is to get a magnemite in emerald and evolve it in emerald and then transfer it to pal park and do a capture show. then level the magneton up in mt. coronet.
one word: cloningUse cloning to clone rare candies (clone Pokemon while it's holding a rare candy) and use them on your Pokemon.
the easiest way to level up your Pokemon to put it in daycare Pokemon will level up soon.Trade the Pokemon to friend and have it hold a lucky egg it will level up quickly.
have EMERALD en clone a Pokemon with that rare candy
Probably trade with Ruby.
the easiest way for me is to give it exp.share and face the elite 4
The easiest way is to buy a Gameshark, there are cheats pre-set into it.
soodoowoodoo level 30
By using too many cheat codes at once.
The easiest way to get Phanpy is to migrate from Pokemon emerald or ruby :D