Edited by Kristi West
The clue is to first slow them down and then shoot them. My advice is to start in the corner where the enemies enter and build two towers (high on the normal level and medium high on the hard one), one tower with a sludge thrower and one tower with a sling shot or poison thrower depending on what you can afford.
After you’ve killed some enemies, build a tall tower a little further away so that the enemies walk straight at it.
Collect a few points and then build a poison-thrower closest to the biscuits. Kill some more and build a “slow-downer” there.
Build towers with water-balloons trough the diagonal as well.