Reddit came to the rescue. Copying from my Reddit post about it:
The Tonic of Mighty Offense recipe can be found in a Cabin atop a hill in the Storm Coast, in the Apostate's Landing area, due northeast of the Small Grove Camp (You'll have to do a good bit of climbing to get to it). There's a locked door inside that cabin. To get the key for it, you have to kill a crazy apostate mage (He's extra strong, you can't miss him) who spawns somewhere in the vicinity of the Driftwood Margins camp, in the hills nearby it (away from the coast). This is somewhat near the Red Templar cave in the far southwest of the map, for reference. Picking up the key gives you a quest to open the door. Downstairs from the newly opened door is a chest, and inside the chest is the recipe.