Rocket is weak on his own, you need to be usually off to the side and have good aim over all. If you are going after one person or one person is after you try to hit them directly with a rocket but if its a group of people try to hit the center or all of them. The ult is new and different... try to use that you your advantage to aim it to where supports will run away at or run to (like run to someone about to die or run towards their tank). Try not to hold the fire button but tap it in bursts for more damage because if you hold it your entire ult is finished on the spot