Okay heres how I do it.
I took a look at the top maps in the top 100 and I noticed that the main trick is to have your squirt towers together and in a way so that the creeps make multiple trips(3-4) in its range of fire.
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All you really need to build is pellet towers as walls, frost towers, squirt towers, and anti-air. minimum of 2 frost towers should be placed in the north and west for the air boss.
frost towers should be placed so that it maximises the time the creeps are slowed, they should cross each frost tower twice minimum, and they should be hit again towards the end of when the 1st frost hit is wearing off.
Mazes should be built so there is a minimum amount of width for the creeps to move through, this 'milks' the squirt towers range.
Level 48 should not be released early, but level 49 should be, so that the squirt towers can dedicate there concerntration on the flying boss and then the level 48 boss after level 49 has been defeated.
You only need to build pellet towers until the 1st round of flying creeps comes, then you should build 1 squirt tower(upgraded as much as you can), then an upgraded anti-air by the 2nd round of flying creeps.
On normal there should always be 3 round of creeps on the screen at once, and on hard, 2 rounds.
If you have just built a fully upgraded squirt tower(forget what there called) then you can afford to be more aggresive and send an extra round a little early then what you normally would.