This is fairly straightforward, load the Port and collect three gas cans. Doing this will trigger the tank to spawn. If you are playing with friends make sure no one fires at the tank. Head back towards the bridge and allow the other survivors to kill the tank for you.
Two Controller Method
If you are doing this on your own, this will be a little harder to do but you will get the same results. Load up a single player campaign with two controllers. Once you start the level change the difficulty to expert and kill the two AI survivors (make sure you change the difficulty back to easy after you kill the AI).
Move both players down to the bridge and decide which one you want to use to collect the gas cans. Collect three gas cans and the tank will spawn. Once he spawns make your way back to the bridge and allow the survivors to kill the tank for you.
With this method you need to keep the second controller active. Make sure this account go idle as the AI will shoot the tank. Wedging something under the so the player keeps moving seems to work best.