The easiest way to train weaker Pokemon is with an Exp. Share - just give it to the weak Pokemon of choice and battle strong trainers, the Elite Four recommended, to gain levels rapidly.
Or, you could just leave them in the daycare and do random things; you'll find that levels increase quickly when you're busy.
If you don't have the Exp.Share, you can put the weakest pokemon first in your party. Then, when a battle begins, switch him out and use a strong pokemon, so the weak pokemon will receive half the exp of the battle.
1] get an exp. share
2] then go to 1 of the following place
- mt. ember (outside the spa)
- sevault canyon (outside the chansey dance house)
3] use vs seeker outside the spa/chansey dance house
4] fight the trainer there
5] you can heal your pokemon at the spa/chansey dance house
Or, you could go to the couple on fishing road, with Nodiking and queen. If you have a good water Poke with surf, you'll be watching it's level increase!
up to lvl 45-50 i use e4 with exp share then go sevault canyon n battle trainers around there til the pokemon im training r high enough to battle e4 by themselves
What I do is I give the pokemon I'm training the EXP share and go to Seavault Canyon to train. There are a lot of strong wild pokemon and trainers there.