Straight Six

Easy to solo at level 120. Get the boss's health down a bit (probably without dots or else you might accidentally kill him), then he'll shoot out 6 sparks. Don't kill the sparks! Stop all damage and get the hell off of the main circle area the boss is on, because as soon as those sparks hit the edge the floor is going to disappear and you'll fall to your death. Once the sparks hit the edge, they'll transform into the Focus pillar things that you actually need to destroy for the achievement. Run around the ring and kill them all with whatever instant-cast ranged spells you have. I play a DK so I don't have any substantial speed boost or long range spells, and at level 120 even my Howling Blast killed them in one hit. But I just ran around and did Howling Blast on each of the pillars, then went and killed the boss and got the achievement. Note that the boss will go through this phase with the 6 sparks multiple times if you don't kill him, so if you think you messed it up a bit the first time, but sit there for a while and wait for the boss to do it again - you can get in multiple attempts in a single fight... or you can choose to run off the edge when the floor disappears and kill yourself if you really want to restart the encounter.

I just did this and took far longer than 10s between the first and last one. I believe that the 10s timer resets in between each one. I was on a prot warrior and it took me about 20-30s to get to the last one. Killed the boss and it popped. The wording of the achieve is a bit misleading.

Getting this achievement is actually surprisingly "easy" because this is actually the optimal way to handle that phase! If one does not kill the Focuses within 10 sec of each other then expect a very rough transition due to the sheer number of adds that spawn.