This small island has a couple high level Sniper Towers guarded by Machine Guns and a Flamethrower. Further back is a Rocket Launcher which can be a serious issue for Zookas and Riflemen.
If running Riflemen and Zookas, take out the Rocket Launcher with your Artillery.
Use Shock Bombs to disable the Flamethrower and Machine Guns. You should be able to hit 3 or 4 defenses with a single shock bomb.