I had some trouble the first time I played, but the second time I realized the power of overlapping hurt zones and slow zones, with maybe a few walls for good measure.
After harvesting a bit of mana, things snowballed, and I practically carpeted the whole floor with slow/hurty areas, concentrated around the enemy spawn points. With walls around the crystal to protect stray snowballs from jumping snowmen, I had a pretty much undefeatable setup.
I eventually let the snowmen kill me, though, 'cause I wanted to see my stats at the end. 322 monsters, 228 snowmen, 5 bombs, 611 mana, 1211 points.
I enjoyed the game! I liked the variety of powers you could use in defense of the crystal. If you were to continue on it, you'd need to limit strategies like this, though.