This is my personal best way to rank up fast
I completed as many daily quest to get my multiplier up as high as possible x2.00 would be best. Then go to the multiplayer portal and set up a custom match. The settings that i used that worked best for me where. Gnome bombSandy Sandsdifficulty easy
(doesn't really matters only affect coins)set plants to Revenge of the Cobsset zombies to classic zombies
Do not add crazy setting this will decrease the multiplier.
I used the sunflower because you wont get many kills i focused mainly on assists and revives.
I had a multiplier of x1,75 and this gave me 43xp per assist and kill and 87xp per revive.
With a multiplier of x2 this should be 50xp per assist and kill and 100xp per revive.
I was able to go from level 1 to 10 in 3 to 4 matches and could get around 2 promotions in an hour.
So it will be slightly quiker when you have the x2.00 multiplier.
here is an option to use when playing with the zombies.Rogue Wolf
Im more a zombie person I would change a few things for the solution. For zombies choose imps only, plants set to sunflowers only and choose a scientist. As with solution same map etc and make reviving and defusing bombs your priority. The imps have no health and you get the quick revive with the scientist.
With 2.00 i was able to go from 1 to half way through level 8 in one game.