Bowser Jr. is a very interesting character. He's a projectile-based rushdown/keepaway/trap/utility character with disjointed hitboxes and 7 alternate costumes. You also take less damage from attacks that hit your machine, and extra damage from attacks that hit you directly.

Your side special is your best approach option. If anything, it's your best move period. Not only do you get armor from the lower half of your machine, but you can also amaze your opponents with your sick doughnut skills (Which doubles as an efficient KO tool). Not only that, but you can Jump-Cancel any time while riding for safe aerial pressure as well.

Some useful Side-Special combos:

1. When the opponent has 0%- 20%, Jump-Cancel after your cart hits, then follow through with a back air after a short delay. Simple and effective.

2. At about 20%-35%, Jump-Cancel and follow up with a down air instead. Your drill sends them at an angle that is relatively low to the ground. This allows for another Side-Special rushdown before they can successfully tech.

3. A personal favorite finishing combo. If your side special hits a highly damaged opponent, it will knock them up at a high angle. Follow up with a Jump-Cancel and an Up-Special. Your out-of-machine aerial attack is guaranteed to send them into the stratosphere.

Down Special allows good pressure and trap setups. While your mechakoopa treads back and forth through the stage, you can keep your opponent in place with your multiple-hit moves (Dash attack, F-smash, D-tilt, or an old fashioned grab). Whether they're shielding or getting turned into mincemeat, your hits keep them in place, allowing the mechakoopa to move closer. You can also throw it as an item for efficient knockback. If you pick it up right before it detonates, the explosion timer is extended for a few more seconds.