Thank you guys so much for the response. I've finally completed the achievement forming my own squad though it took me about 4-5 hours to gather enough players (30) and do the achievement over and over again since people left after every failed attempt.
One thing to mention for people that might come and look through this thread: The requirement for this achievement is defeating all 3 bosses within six minutes, hence the name. However you need to kill them all before the timer hits 9:59 which starts at 15:00 mins. That is because there is a 1 minute wait between the time bosses spawn and are vulnerable/available for attack before that 15:00 countdown starts. The achievements counts that 1 minute wait as the start of the event which leaves you needing to kill the bosses in 5 minutes actually.