I have plenty of experience opening up and restoring wavebirds, so I can try to help! First and foremost, you’ll definitely need a triwing screwdriver to open up any gamecube controller. Next, it sounds like if it’s stuck, it might be the spring that’s the issue, but there’s no way to tell without having some pictures of the inside. If it’s the shoulder buttons in particular, once you’ve used the triwing to open the outer shell, the shoulder buttons have additional phillips head screws, so you’ll need a small enough phillips screwdriver for those too!