For people who prefer Offense over Defense
Make sure you multiplier is 2.00 before you start to make leveling up as quick as possible. I used Rose
for this because of her lock on ability made it extremely easy to get kills as quickly as possible. The settings I used were
Mode: Gardens & Graveyards
Map: Great White North
Plants: Sunflowers of Happiness
Zombies: Imp Invasion
No crazy multipliers as it will drop your multiplier below 2.00
The basic strategy is stand in front of the hill on the first objective and the imps will funnel right into the middle giving you a clear and easy shot to kill them quickly. With Roses lock on ability, slow and goat you should be able to rack up 50-60 kills per objective. Any upgrades you unlock later help a little but with all the sunflowers around you should almost never die. I would suggest stopping them if possible, the second area is also a great place but a lot slower!
All numbers at X 2.00
50xp per kill, 50 per assist, 50 per goatify, 50 for saving an ally (you get this often)
I was able to promote my character after every 2 games. So every 10-12 minutes thats one promotion!
EDIT: As of 3-8-2016 imps had their health buffed and rose recieved a nerf. Not nearly as great but Gnome bomb with rose works well too.