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Operation Machine Gun
Aside from the gatling, you can also use rockets. You will be defending the whole time. You will be attacked from the air and from the ground. There are different bonuses awaiting you in the crates scattered around the field. Try to keep up the fight for as long as you want. Firstly get through the training level and then onwards to the battlefield! Utilize med-kits, since they will allow you to keep up the fight much longer.