It's to late on this costume but a great way to do battle damage is to sprinkle rock salt on the area to be painted, spray the salt with water, let it dry, and then spray on the color. Then once the paint dries you can just chip the salt off (a stiff brush works well for this) and the paint comes with it. So in this case you would put on a coat of silver (if you wanted it to look like metal), let it dry, sprinkle and spray salt, let it dry, then spray on the green, let it dry, and lastly chip off the salt.

That's a relatively easy way to do things with a very low chance of coming out badly. Though there are a ton of different ways to distress things and I would definitely suggest trying them out one of these days. It's a ton of fun to do and so worth it in the end.

All that being said it's looking great and I can't wait to see some pictures of the costume when it's 100%.