Give it an EXP Share and take 5 other strong pokemon. Then go through the Elite Four. I can guaratee that it will be at least Level 20 when you finish.
What I usually do is take my strongest Pokemon and go to a route with high level Pokemon, and put the level 1 in the first slot. When you get into a battle, immediately switch to the stronger Pokemon and battle. The level 1 will gain experience extremely quickly.
route 205 for Pokemon diamond, pearl and platinum. the trainers have Pokemon from level 10 to level 17
Take high level Pokemon that know attacks, then attack opponents Pokemon and hope for the best :-)
If you aren't to the Elite Four yet I would suggest either SnowPoint city or somewhere around there. Either that of in the water depending on your pokemon, for best results use a rod.
undergruond mineing in walls
in the elite 4 It depends if you're playing gold/silver,/ fire red/ Saphire,/ Enerald,/ Ruby/ leaf green, or diamond/pearl
elite 4
route 205 for Pokemon diamond, pearl and platinum. the trainers have Pokemon from level 10 to level 17
Route 201 or 202 or 203
the place where you can breed Pokemon next to heart-home city, that's where i go
i trained mine at mt.cornet
elite 4
An area with grass Pokemon would be best.
Eterna Forest.
You need a high level pokemon with the ability pickup Pachiritsu would be your best pokemon
At the Pokemon league.
I don't think it's possible to get a cranidos in Pearl.
the best Pokemon you can catch in Pokemon pearl is heatran