If you're like me an just love to Pokemon battle for fun and is crazy enough to bring a Pachirisu into battle...here are a few basic tips.
1) Play to your pokemon's strengths. Little guys like Pachirisu had weak attack but make it up for having somewhat higher then normal speed, so EV train those strengths. In addition breed your pokemon to have high IV values. The guy in Nimbasa City subway should tell you about your pokemon's IV values.
2) Use a moveset that makes up for the lack of strength....moves like flail and super fang. In addition use moves of the same type of as your pokemon for "Same Type Attack Bonus"
3) Choose your item carefully....if you Pokemon is using Flail, why not a Focus Band or use Quick Claw so you can use moves like Super Fang first so a secondary pokemon can deliver the finishing blow.
4) Have fun and maybe the impossible will happen and Pachirisu and friends will win.