Build a squirt tower in the entry way, and upgrade it typhoon as soon as possible. Use cheap turrets to create isles so they have to keep going past the typhoon tower. Instant butchery!
The best way to get past 20 is to upgrade the towers pronto, and not to forget to get some airstrike towers up before the first wave... I like playing the "fun" levels. Anyone made the "Best Players" list yet?