Giving credit to Queegum, this is a variation off of his.
Do the first challenge level on insane difficulty
Timmy (devastating attack)
Jimmy (devastating attack)
Pip (devastating attack)
Kenny (need for more coins)
In the first open spot: Upgraded plasma tower
In the second open spot: Upgraded refrigerator tower
In the third open spot: Upgraded plasma tower
Remaining spots: snow to slow movement for stragglers
Leave the all of the players on the first square so they can work towards their special. You will have no problem on the first couple rounds, but after that I would use one character's special attack per wave to take it out.
You need to pay attention to Kenny and use his special, then use another person's so that you can amass as many coins as possible (they go towards the round end bonus). Be patient so to get through as much of the wave as possible and then use a character's special to finish it. That should do it, very simple and easy with this method.