Train it as you train all pokemon.
Get into a double battle have your weak Pokemon go out with a really strong Pokemon also have your weak Pokemon either hold exp share or lucky egg defeat the trainers and your Pokemon will level up! Even faster way? Have the weak Pokemon your training be traded to you from another game its experience will be boosted and will get lots more exp at the end of battles just as long as you copy the above steps your fine.
The Pokemon that are weak to bug Pokemon are Pokemon that are the grass, poison, or psychic type.
Water type Pokemon are weak to electric type Pokemon.
one that is the same type of Pokemon weak levels weak moves and some one who blows at playing
Train them............
you train them
take a weak pokemon to the elite four
with a lucky egg or with cheats -.-
fight gym leaders that are weak against your type
you can train it at some weak places like where you start your adventure or you can let it hold a exp.share and fight pokemon with another pokemon in your party.
Use the EXP SHARE Method, or you can train from weak Pokemon and getting harder.
Personally, I would train a Bird-Type Pokemon at a place where those types of Pokemon is very effective to it. Like Fighting-Types. They are weak to Bird-Types. So, I would train it there!
i would say Larvesta. It may look weak but its really strong when you train it up
train ur strongest to lvl 100 then give the weak Pokemon experience share then u can battle with ur lvl 100 and still get ur weakest Pokemon exp so u should train ur strongest
Wynaut is generally considered to be a good, but weak Pokemon. If you got it in Emerald, you may consider leaving it in the pokeball as it has a weak defense and offense.
let the Pokemon hold the exp. share and train it at route 229 with your best Pokemon