How to go up levels fast

Snipers: so puny your drones
can take them out!

Being a higher level in World Zombination gives you two main advantages:
1) you have access to more raids (you need to be 32 to play in Bogota)
2) your borrowed unit can be a higher level

So how do you get up to a higher level fast?

The short answer: by getting a lot of trophies!

Levels 3 - 20
As soon as you can, play San Francisco on Hard difficulty. San Francisco hard only has snipers, which can be killed by your drones. All you need is an infector or reanimator and the "guild Goliath" (or any other borrowed tank or explosive). By playing on Hard, rather than Normal, you'll go up levels faster and also get lots of brains.

(I'd also recommend making your second team early as you will be able to get through the campaign faster, which also helps you to go up levels).

When you finish playing with your main teams or farming, get into the habit of quickfighting any left over units not on cooldown. The trophies you get in a place like Vancouver on normal difficulty actually add up quite quickly and help you go up levels fast!

Levels 20 - 50
Once you are around level 20, try using the NEKO method (borrowed explosive on expert difficulty in raids/faction wars). It is a good way of playing for as long as you want and getting trophies and DNA as well.

Punks are great at attacking melee units

When you are around 22-25, try brain farming in Lima Hard with borrowed delinquents/punks/hooligans only. You only need a shot of the flare to direct them to take out the Spec Ops guys. You can also play in Lima expert, but you probably won't win every time (I found I won about half the time, using a level 25 account while Jifferlaar has found once you are level 40, you'll win Expert about 80% of the time).

Continue to quick fight any extra units and you'll be at level 50 before you know it!