I had similar problems, not getting credit for perfect hits for the achievement for the quest. I had 3 or 4 perfect hits with Normal Attack that weren't credited. I read through a bunch of the posts and found at least one person who had gotten it with a different attack. I switched to easy in hopes it would increase the sweet spot (unsure if it did) and then managed to get a perfect shot with Headshot and was indeed credited with the achievement. My interpretation is that Normal Shot isn't being counted as an 'ability' and you'll need to score the perfect hit with a different attack.