How I got diamond stars (mainly later levels which are harder) :
Build two archer towers and three totem's of accuracy (making sure both archer towers are in range). Upgrade them both as much as possible into frost towers and then max out the stun (Ignore the iceberg attack), keep on building another 2-3 frost archer towers (in range of the boosts) all with stuns. Once this is done you should be doing fairly well and can build other things (I prefer magic towers and the mana steal).
If at any point mobs are close to the children (mainly at the start of game or when swarmed), summon zombies, if you're in a real bad state do fireball and chain lighting but these won't be needed often.
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