Sun of a....

..Lich!

For an actual strategy...

Either 1 tank 4 dps or 4 dps 1 healer. We did it with 1 tank 4 dps. DPS need to be at least 12k, tank around 5-6k. Bloodlust needed obviously.

Now, this fight has a bit of RNG. The boss has 100 focus, he use focus each time he use an ability (fire tornadoes, heroic leap, orb summoning, etc. Basically everything except auto attack). So the trick is to hope that he actually use abilities that can be interrupted, so he doesn't lose focus on abilities that can't be interrupted. Also, if you interrupt one of his ability with Counterspell, he will not lose any focus for 8 seconds (since he's interrupted for that long). Interrupts recommending are Counterspell, Shield Bash and Spell Lock (from a felhunter).

Another trick is to tank him in the corner of the room, so he spend more time walking to the middle to begin recharging. Position yourself accordingly so you don't lose dps time moving.

Note: to get the achievement, you simply have to kill him before he totally complete his AoE when he's charging. It's very easy to see, he gain a buff when he recharges. If that buff runs out, you've failed the achievement.

in 6.0 just zerg it, piece of cake.