Much of the Pikachu players I've come across act like scrubs. All they do is use Down+B (Thunder) and Forward+B (Skull Bash). Or they spam their neutral B (Thunderbolt). I don't play as Pikachu, so don't ask how I know the names of these. Let's just say I've bumped into a few Pikachus since March 9, 2008.
I think we ought to create a Match-Up list for Samus. You are probably well-known by now in this forum for creating, "How do I defeat -insert Brawl character here-?" The only way to help answer these questions or offer tips and the difficultly level would be by creating a thread and having it stickied.
Anyway, keep in mind that characters act differently on different stages. They have to, because some work better than others on certain stages. For example, a level 9 CPU Peach can go psycho on you at Final Destination, because she can fly back, and she likes to hover while using her ninja kicks on you. However, if you go to another stage, say, Onett, she's going to act differently. The same applies for human players. If you are fighting against Olimar, and you're at Aero Dive, guess what? You'll likely have a greater chance at winning.
So, if you go to a stage where there are platforms, like Battlefield, and Pikachu decides to use Thunder, it's not going to work if you're under the platforms, and he summons it. (Pikachu's Thunder has nothing on you if you're at the Temple, and you're in the basement.) He can be under them too, but still miss. Note that Pikachu can jump in the air, move left or right, and perform Thunder, causing it to miss him, but hitting the location from which it was summoned.
There is more than to defeating your opponent. You got to ask yourself if the stage you're on is in his favor or not. If it's not, take his disadvantage and use it against him. Other than that, I'd recommend using Homing Missiles. You don't have to be fancy with the short hop Homing Missile cancel. Brawl is not about being fancy. It's about survival and winning. Other than that, take up a battle with Pikachu in Training Mode and see what he has to offer. Then go from there and use whatever you've found to be effective against him.