So, this achievement is just huge bullshit. It screams lack of QA on blizzard's side. Been chasing Guitarist for 20-25 tries now, and nothing. I of course read and tried all there is on wowhead, but so no luck.
I got the 3 other criterias easily enough in the 1st week (2nd now), but now still none.
Everytime I whisper people who got it after boss kill they come with vague answers, or tactics I already did.
I tried get high score (even after the nerf), do different tactics etc...but nope. Im thinking if somebody else is also going for Guitar buff, the game roll a dice on who gets it if more than 1 player fills criteria. As in, only one gets a buff pr. kill.
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Sigh, like right now...I got hit once by banshee walls and missed 2 lights....didnt get anything. And the one who got guitarist said he didnt get hit by any walls, but I saw he missed many lights...Like fuck this :/
Edit: After like...25ish try I finally got it. Got hit twice by wall's, popped CD on both, only choose 1 hit pr. spwan of wall, missed all others 100% - Also missed first two stars in + 2 stars randomly later as ghouls we're dying. So in total 4 missed stars + 2 hits.