The best moveset I could find for Mach Rider. I'm a supporter.
Neutral Special
Machine Gun Mach Rider takes out his machine gun and starts firing. If the special button is not held down, Mach Rider will put it away after firing a few shots. If the Special button is held down, he will continuously fire bullets. Every bullet does about 1% damage, though this can add up quickly.
Side Special Mach Bike In a similar manner to Wario's side special, Mach Rider hops on his bike and rides around on the stage. However, unlike Wario's Bike, Mach Rider's does not drop onto the stage. Instead, Mach Rider puts it away, making this move a bit harder to use for recovery. Every time you ram into someone with the bike, it deals about 9 - 10% damage.
Up Special Mach Flash Mach Rider leaps into the air, leg forwards. This is Mach Rider's main recovery move, allowing for decent vertical and somewhat horizontal recovery. It can also hit opponents multiple times, and it has startup invincibility. Damage varies because of the number of times Mach Rider can hit someone with this move. It can cause up to 14% total.
Down Special Mach Kick Mach Rider starts kicking very rapidly, dealing multiple hits to opponents who are caught in the fury. Each kick doesn't do a whole lot, though like the Machine Gun, the damage can add up.
Final Smash Mach Assault Mach Rider hops onto his trusty Mach Bike, equipped with the Machine Gun. Mach Rider can then run around the stage, dealing decent damage with each ram from the bike, though it can also rack up even more damage by firing the machine gun. Mach Rider can also perform a wheelie while performing his final smash.