If you want a fair-quality video, you'll need a few things:
A good camera with fair quality could cost you a couple-hundred dollars. You could find one on Ebay for like, 70 bucks though, which is decent. 12-16 Megapixels and 720p-1080p resolution will make your recorded battle look REALLY good!
Camera Tripod / makeshift stand:
Use a camera tripod or some kind of stand made of boxes to keep your camera from shaking in your hand. This also grants you use of both hands. This is probably the most important part of making your video look good.
lamps / lighting (Optional):
Lamps are a great way of giving light, in case your shot is filming the whole DS, rather than just the screens. This is optional, but is worth a mention.
If you want the cheapest financial investment possible, go to a home appliance store or an office supplies depot (Staples, for example), and buy a little desk lamp. Take a piece of paper and tape it over the bulb, to drown out some of the light. Do a few practice shots with the light in different places in the room until you find a nice angle. Lighting is essential to making a nice video, as it makes them easierto see,and lighting can even remove the bars and flickering on the screen, if done right. Yay!