Here's a video showing how I got 460,000 score on insane difficulty.
The general idea is to use Waterfall (water/nature) towers to boost your strong towers' DPS by 90%
You eventually get Invocation (nature/light/earth) towers to amplify damage further. Percentage increases in amplification or buffs are very strong when you have a few very strong towers.
You will end up with 3 pure element towers that can deal a lot of damage.