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I have an idea that the Rebels start at the vortigaunt camp, in that pit/training area with the hanging Combine soldier, and the antlion pit where they can spawn from. They can prepare, load their weapons etc. After they set-up, that wooden gate/pile of wood that acts like a gate springs up and you can proceed to the beach. You fight small groups of antlions on the way, and this forces the Rebels to work together to not get overwhelmed.
After they reach the right side of the beach, Combine soldiers spawn on the entire beach and in the bunkers, and are under attack by antlions, which also spawn and act as a common enemy.
So, the basic premise is, the Rebels have to reach that far bunker WAY in the distance(shown in the image, top right corner) While fighting antlions and Combine. The Combine must stop them. Pretty chaotic, intense, and downright awesome! Ammo and health are in the bunkers and on certain points of the beach.
The Rebels would be taking heavy fire from the bunkers and the antlions, but there are supposed to be a ton of antlions, hopefully making it hard for both sides. ALSO: the Overwatch CANNOT spawn additional troops. So, this would prevent him from massing them up on one spot, and would strongly encourage strategy and tactical decisions.