I remember this one as it was the only one I had to make notes on to complete. Luckily for you I still have my notes!
Facts: you have 800 DU to build with, can only build towers, and must have 10 different ones on the map.
My Solution: build all towers near the crystal and use a frostbite tower with frosty power, destruction, and deadly strikes shards, then def rage, def power mods(this will buff everything!)
I went with 6X Earth shatter for main damage (360 DU) and 3x Cannon towers (90 DU) = 450 DU to have 2xearth shatter and one cannon for each lane. Leaving 350 DU to build