I recently came across the bugged Quest "The Chain Gun and Me" in the Winterguard Mausoleum in Dragonblight. After an hour or so of messing around with it I managed to find a way to complete the quest.
When the 7th legion soldier reaches the stairs, they stop at the bottom of them a few yards away from you, they stand still for a few seconds and then they vanish without giving you quest credit. The way around this is to call the soldier out like you're supposed to, spam shoot the ghouls until the soldier reaches the spot where they get stuck and then quickly jump in and out of your cannon. This will unstick the soldier and he will run up the stairs and stop next to your cannon, giving you one of your 8 rescues to complete the quest. I hope this helps a few people.
TLDR: When the soldier gets stuck at the bottom of the stairs, jump in and out of your cannon.