If your going with a low level go to where you typically train and do this. Put the weak pokemon up on the first slot and as soon as possible switch pokemon as it will gain part of the exp.
Exp.share training Is my best recommendation so you should go with it and rare candies are an ABSOLUTE no because they don't raise stats by much.
There are to ways I know. use exp. share, the lucky egg or rare candies. The exp. share can be given to any pokemon in the party and will gain some exp. even if it didn't battle (god for low level). The lucky egg is a exp. multplier. It raises exp. gianed by 1.5. Rare candies raise the level of one pokemon by 1, but as the name states are hard to get.