Ok this is how I did it.
-Grab the keys and unlock the cuffs
(Zombie sheriff attacks you fall)
-Crawl backwards, look to the right grab shotgun shell
-Look to the left (grab shotgun and load it)
(Lee drops shell)
-Grab shell and load gun again
-Hurry and aim it at the zombie sheriff and press A (or I believe X on the PS3)
then BOOM! Shotgun to the face. Zombie dead.