Mastering this fight is all about positioning. If everyone is positioned properly then Kargath's Blade Dance ability does minimal damage. When you first engage him, the tank has to very quickly get into position tanking Kargath in the very middle of the platform. This is crucial, so the tank should be diligent about his position. It is recommended he set his camera distance to max and look down on his head to make sure hes in the very center.
The next most important positioning is done by the healer. The healer should follow the tank in, taking up residence in one of the corners near the entrance to the platform. They should stand at maximum distance from the tank.
1 or 2 ranged DPS needs to take up positions in the corners flanking the healer. If everyone is positioned correctly, the healer should have no problem reaching the tank and the two corner DPS classes. The last DPSer, preferably a melee, needs to be given the job of guarding the entrance to the platform from the rushing mobs. After 20 seconds the first adds will show up, and that is when their job switches to purely managing adds. The other DPS should help out to take them down quickly, then return to damaging Kargath. If an add slips by and goes for the healer, the guard player should stay in position and let ranged DPS take care of it. The tank should never leave the center of the room.
By following these rules, you should have absolutely no problem beating Kargath on normal or on heroic mode. Incidently heroic is exactly the same, only with higher damage from melee attacks and Blade Dance.