Well, if your Marshadow's nature decreases it's Atk, and its IV's are for SpAtk, utilize the SpAtk. With it's already great Spd stat as well as +Spd from the nature, you may make it a special sweeper.
It knows some good special STAB moves such as Shadow Ball and Focus Blast, and maybe have the rest of the moves be something like Toxic and Protect.
On the other hand, you can just EV-train the Atk stat and take advantage of it's ability Technician, and give it moves like Shadow Sneak and other STAB moves. Bulk Up may also be an option if you want to "repair" its Atk stat.
Hope I helped! :)