In order to defeat all the monsters in the Brewery, you must first learn how to enter the Raging Boar Inn, one of the pubs that can be found inside Cawdor Keep. In order to enter the Raging Boar Inn you must first find the tracks that show you where the secret door is. The top leftmost vat can be entered from the east if you track something up to it and then use an adequately powerful Detect skill, True Seeing spell, or some item that grants either of those two. The left wall, top to bottom, conceals the Raging Boar Inn.