Originally Posted by
Keoren
Talking about personal experiences, I completed this achievement three times and only got it awarded on the third. One of our guesses was that, because on the two first times we just completed the requirements on random dances during which the two bosses' phases were slightly off sync, the achievement might be running an internal timer and the strikes were too far apart. We got the achievement normally when completing the dance on both the first one and the first one of the titan gas phase, just to be on the safe side.