Here's really the key to killing Gruul. Just don't die. Yeah, I may have oversimplified just a tad. But yes, last night we killed Gruul the Dragonkiller. The shatters really make for an interesting battle. Every 45 seconds or so, you get thrown up into the air in a random direction, and then have about 5 seconds of increasingly slowed movement to get into a position away from other people until you are stoned (no, not that way), and you can't move for 5 seconds. Then, if you've done a great job, nobody's within 15 yards of you and you won't take any damage. If there's too many people next to you (or just 1 too close), you'll die.
There's other mechanics to the fight as well. But I found a great spot where I could run against the side wall just before the Ground Slam (which throws you in the air). Running into the wall usually would result in me just standing in the same place. And as long as nobody else was thrown into me, I wouldn't take any damage at all.
At any rate, the exciting part is downing him. But also is exciting is the loot that is dropped. Our guild got the [Leggings of the Fallen Hero], the [Leggings of the Fallen Champion], [Shuriken of Negation], and something else that I really don't remember. I thought I had my T4 leggings in the bag when I rolled a 96; but, Encore (one of our warlocks) pulled through with a 100. They would have been nice to have, but I'm a patient man. Just one of those things that because I rolled so high, I thought it was mine. But, that's the luck of the roll.