This achievement is ridiculous. If you want everyone in your raid to get the achievement, it's going to take a very long time. For example, a raid of 11 people (10 people casting the prism, one person every 6 seconds) would take 5 minutes to get the achievement for one person. This means at the very least it would take 55 minutes for everyone to get their achieve, assuming no mistakes in the casting rotation.
The buff from channeling the prism lasts 10 seconds. With 11 people in raid, the person being targeted will need a person to cast the prism at him once every 6-9 seconds. Repeat 50 times and get the achievement (or fail halfway, more likely). Plus, if not everyone has all 50 charges on their prism, even if everyone casts their prism perfectly all 549 times (assuming only one person in your group used a single charge of their prism), one unlucky person is going to be leaving that raid without the achievement.
I just tried to do this, and it was as much of a nightmare as anticipated.
After a failure due to 1 person out of 40 missing it people at the back of the line realised it was literally impossible for them to get the achievement and they ragequit having wasted 30 minutes of their life.
Got the leader 50 and he declared "Oh, I just realised I have band practice! Thanks guys" and left like a scumbag. Group tried to struggle on, but disbanded soon after, with 40 people having wasted their beam charges, making it impossible for them to even attempt it for a day because of 1 selfish jerk.
Thank you Blizzard for this gaming experience.
Much of the info in this posted lifted from wowhead:
http://www.wowhead.com/achievement=9...really-love-me