Reach the base as fast as possible. Start from long distance, shooting the first three targets in the front. While avoiding enemy fire, use your boosters to get to the next three targets of the left side of the base. Once they are destroyed, use your boosters to jump the first hanger to your right. The Guntanks will not harm you. Watch out for GM, and use your claws, axe, etc. to hack away at the hanger until it explodes. This will take some time. Then, boost to the next hanger and repeat the process, still watching out for GM. Then, do the same to the last hanger. Once all hangers are gone, kill the target to the far right of the base. At this point, kill as many of the little tanks that do nothing. Killing them will ensure an "S" rank. Destroying about six of them should be sufficient. Then, kill the target to the far left.
To get an S+ rank with Zaku II, get to the left side of the base as fast as you can. This way, the GMs cannot get to you. Take out the four cannons on the left side (ignore the Guntank, it will not hurt you). Then, use the auxiliary attack (Cracker) on the hangers. Two Crackers should be enough to destroy a hanger. Destroy the last four cannons. This should take about two minutes to complete. Note: This can be done with anyone who uses the Cracker auxiliary attack.