I tried specializing all my points into a single spell tree and then tested them on the same couple of levels. I learned that the Ice is MUCH more powerful than the other two. This may just be my play-style, but ice was way OP.
A final objective might make it more interesting for a 5 minute game. Also, you might want to inflate the cost of the sps items to make it exponential rather than linear. (typical ideal game suggestions) Good luck!