You can lower it to 1 specific device only, the question is... is it necessary? Generally all ports will be hitting that device, outbound traffic isn't affected when the firewall is up, it's only really getting in to your network where it needs to be configured specifically. It's a hardware firewall on the router that's a naturally occurring NAT firewall, it's not quite the same as a software firewall such as Norton.
Regardless, to lower it, the device in question has to be in DHCP (obtain an IP automatically) as DMZ will forward on the peer address of the router to that device.
Navigate to the router via 192.168.1.254 in your address bar, go to Settings> Firewall> Allow Applications, Pinholes & DMZ Mode. A list of your connected computers will show, select the device you want to lower the firewall to... the page will refresh and it will not say You have chosen yourPC. Scroll down to the bottom and check the Allow all Applications radio. Click save and it'll ask for your router password (the 12-digit serial number by default on a sticker on the base of the router). Once it's complete, you will need to refresh your IP on the PC... rebooting the device is the easiest option.
