Ready, Set, Snap!

The trick to this achivement is that the explanation sucks.
It is quite easy with voice chat.
Make everyone stand on the edge of the pull. call 3, 2, 1 but run on 1.
Then every player can break it within the 3 secs of the first player. You do not need to break it within 3 secs of it casting.

If you sufficiently outgear this fight, a slightly easier way to do it is to focus Vaelgor and just heal through the damage buff from [spell=1270189] and [spell=1270250]. You only need to survive one round of Nullsnap to get credit, while every subsequent cast gives you a chance to fail it. The faster you kill Vaelgor, the fewer rounds you have to do correctly--at current gear levels it's pretty easy to kill him after two on Normal, and with better gear it should be doable with only one.

My group found it easiest to just have everyone try to break the tether immediately (helped with stampeding roar/ wind rush totem or whatever else you have available). Just make sure everyone is prepared to move quickly (the cast is right after the Nullbeam channel).

Can confirm, this one CAN be completed with a raid smaller than 10.