Since it's apparently not obvious enough, YOU ARE SUPPOSED TO LOSE. THERE IS NO WIN CONDITION. Before starting, make sure to read the Readme inside the Map Folder in order to get the Map working. Also, welcome to the first China Coop Mission for Contra! You can prove here, how much you care about every single Unit and also your Teammate. If you don't, it's going to be harder than usual.
Read the Readme File inside the Map Folder for more Details.