Just did the dungeon again. Immediately after Mai Trin and Horrik came from the 3rd barrage phase, Mai Trin initiated another barrage phase (for a total of 4).
The 4th barrage lasted around the length of the 2nd.
Completed the dungeon fine, but it did cause us to have about 15 seconds left for the 15 minute timer for the achievement.
Gelda Nebilim – Nagare [NGE] – Crystal Desert
http://youtube.com/user/Royblazer