First of all, you should beat the game to unlock all the characters.
For this I went to challenge mode and used 20 Squares.
My character choices would be: Butters, Stan, Cartman, and Craig.
Save Butters and Cartmans specials for when there's LOTS of enemy's on the screen then take 'em out with your snowballs.
Also if there are lots of enemy's on the screen and no one has a special use Craig's as it slows them down recharges fairly quickly.
Now, as for stan, just throw snowballs and use your special in case any emeny's get past you guys. (They shouldn't.)
For the map, just make a snake type thing, go up and down with the walls.
Use lots of fireworks (preferebly upgraded) for Yanagopa, 6th Graders, and Homeless People. You also want some refrigoraters (preferebly upgraded). And then add in any towers you want, as you will a good bit of space to build.
By the end my friends and I earned 2,930,964 points. And it only took about 15-25 minutes. (Yes this way is a little longer, but it's those of you who like a challenge, like me. :D) And GOOD LUCK to you all!