[M. Mellott]

I'm picturing...a small motor (small but fast...remember, F=ma, and if you can't have mass, go with speed) with a pair of wheels driven directly from the motor. The assembly would run along the inside of a pair of rails with thin plates acting as motor mount, wheel mounts, and guides along the rails to keep the hammer aligned for repeatable contact, with stops at both ends for containment. The rails could be integrated into the frame, if you have one, for improved rigidity. You may also be able to wire or cable in place of rails.

I'm not sure which direction the hammer has to go (laterally, vertically, etc.), but another thing you might think about is the direction the hammer travels in relation to your drive train if it has to go laterally (which is what I'm assuming if you're attacking an opponent). When the hammer strikes, there will be an equal and opposite force on your robot. Not knowing how strong your drive train is and using the above hammer concept, if it travels parallel to the drive train, you may want a simple brake on your wheels (or a drive motor that can't back-travel) to resist the opposite force, putting more impact on your opponent. Another option is running it perpendicular to the drive train so the opposite force is parallel to the axes of the wheels, which will offer more resistance without improving the drivetrain (unless you're using something like omni-wheels).

This may all be too elaborate or too crazy, but it's a brainstorming session, right?