the two armies don't come out until you capture the first camp, so before you do that you can set up whatever you want. I put the Shamrock right next to where the Southern tank was going to come up and then took it out by hitting the radiator. For the Northern tank you can run a lancer out there and get above the tank on the cliff above it. You can do both of these before you actually take over the camp or just do one.
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