Now I don't know if this is in the original Brawl or not, but on the Battlefield stage, you can short hop --> tracker missile and Samus will somehow be on top of the platform, whereas just a normal short hop won't even get her atop the platform. You could then fall through or off the stage, another tracker or fast missile, then short hop again on the opposite platform and shoot a tracker missile. This way you can get about 4 missiles off in 5 seconds. Obviously, it only works on stages with platforms. I've only really tested it on battlefield.