On my YouTube channel I've created gameplay walkthrough videos covering this game.
This achievement is obtained during the Secrets of the Restricted Section quest. I began this quest at the tail end of the part 5 video from Professor Fig. The quest is completed in my part 6 video. I'll provide both below.
In this quest you're sneaking into the restricted section with Sebastian Sallow. During this quest he teaches you the spell Disillusionment, which makes you nearly invisible. This is useful for stealth sections or sneaking up on the chests with eyeballs to open them. I won't go into spoilers here, but once you and Sebastian part ways you'll end up in a new area where you fight some Pensieve Sentries (enchanted suits of armor). During the first encounter I noticed a prompt on the enemies. Following the prompt uses an ancient magic attack for big damage. You have a bar that, when sections are full, allows you to do these attacks. After using the ancient magic attacks on the Sentries the achievement should pop (happened at roughly 20:00 in the part 6 video).
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