Research wind generator, Sun Disc Throwers, Sun Cannons, and stone towers. Build a few cannons above your ally to protect him then bridge off your middle geyser close to the enemy island and build a few stone towers. You should usually build stone towers in front of your sun cannons because they have a lot more hits and are inexpensive. Remember to also try and attack from the side.
Periodically check your ally and guard him with stone towers and a few sun cannons because he isn't all that great. Once you have finished off the red enemy you can help your ally to win.