It took a while but I finally beat the Squirt and Frost Only challenge. Now I can do it every time but until then always assumed it was impossible.
The other challenge I can complete regularly is the Survivor mode where it is nothing but Spawn creeps. This starts out easy enough but quickly degenerates into chaos as the spawns start splitting 5 and 6 times half way through!
My latest conquest was the 10K "challenge". Even though it is listed under the Fun modes it is still VERY difficult.
I haven't done the S&F only challenge, but the Spawns I beat everytime.
I can't get past about the 84th level of the 10K challenge. I try, but I'm not able to do it yet.
__________________
I've gone and done it now! I'm on Facebook!!!
Someone sets up a game (various options to choose from, like number of players) and others join. Each gets a separate field to build towers in (not a bunch of different players' towers on the same field). When one player kills the last creep of a wave, the next wave starts for everyone. First one to kill off the last wave wins. There are also points, but it looks like they don't determine the winner. Not sure what they're for!
__________________
Hebrews 13:23 Know ye that our brother Timothy is set at liberty