What would be the best way to lvl up pocheyena to lvl 18 so I can get a..

Best chance is by battling other trainers in-game. Battles with trained Pokemon offer more experience than regular battles.

If all your other Pokemon are at a high level, use the switching out technique in an area with higher level pokemon.

Put poochyena in slot one in your party. Go to an area where there are really strong wild pokemon. Use your strong high level Pokemon in slot 2-6. When the battle starts, use your first turn to switch poochyena out. Win the battle and the experience is shared between the two Pokemon you used. The more Pokemon you use, the more the experience points get shared around, so it is best to try to only use one other Pokemon for battle so each get half.

If you own an 'exp. Share' attach this to your poochyena and battle somewhere there are stronger Pokemon that other strong Pokemon in your team can battle.

Fighting real strong Pokemon can help. Just get Poochyena first in party. Then when you face a Pokemon,switch out with something else,then kill it. Do this for a period of time then there you go. Hope this helps you out.

Well you're almost there.just need 3 more lvls I think so go to your most recent area and keep in first in the party

Keep it the first in yout party then go into the wild. Battle wild Pokemon and trainers until you obtain your goal