Select the Trene City area. When the battle begins, there is a big building directly in the middle of the Arena floor. Boost up to the top of the building and pummel the opponent AC full of rockets or other types of ammunition that can lock on. If you are unable to boost up the entire way, there is a ledge halfway up. Rest there momentarily, then boost the rest of the way. Weapons that are the most effective are the small rockets and/or vertical missiles. There are some enemies that are too fast to do this to; they move to much to get a lock on. If this happens, when you start, boost West and position yourself in between two buildings very close to the border. Most of the enemy ACs will not go behind because they will lose. They will just move back and forth trying to get a fix, but will not approach you if you are very close to the border. Pop in and out from behind the building and shoot various rockets and missiles as the ACs are moving around. It is slightly time consuming, but a very easy way to boost ranks early.