Once in Ehmead Hill you will eventually come across a giant wolf just left of the first save point. This was quite a challenging boss compared to the rest of the game thus far. Equip any poison resist gear and heal up.
The stipulation for the achievement seems to require
YOU to activate the plant and the wolf being stunned because of it. It didn't ever seem to give me the "Great" if he charged in/hit one himself. You can also target the plants manually with
to make it easier. Wait until he comes near and swing.
Once you've achieved this goal, he will be stunned and "Great!" will appear in the center of your screen. After the battle the achievement will pop.