Brace his feet simply means " hold oneself" You can also say hold the sand under my feet Maybe you are passing through a though time and your friend tell you to brace your feet. this means you have to be strong to move on. A solder was the only one in camp when the enemies arrived and he was about to lose the sand under his feet, but when he rememberd that he must survive he brace his feet and face the battle and he eventually won!

To "brace" is to prepare yourself to be moved from hitting something or being hit. If he "braced his feet" then he moved his feet so that it would be more difficult for him to fall over (maybe one knee bent and the other leg stretched behind). In your story he wasn't being moved, but was attacking the door, but even so, he prepared himself the same way. "brace yourself" doesn't specify the body part. You may hear "brace for impact" when an aeroplane is going to crash.