Originally Posted by
ananoon
Might help if you explained what the fail is to the OP.
I'm guessing that the melee on the healer is happening at a fading light. When the tank leaves the twilight realm for fading light, he loses threat. The other tank has to have taunted before he leaves in order to ensure that whoever is next in the table doesn't take a hit.
In normal mode, I used to wait to see the other tank being targeted by ultraxion before hitting heroic will, but in hard mode, it's a lot more dangerous. Sometimes the fading light debuff is very short (3 seconds), so if you wait to see the bosses target change, it may be too late to hit the button, considering latencies.
Just make sure your taunts taunt immediately when fading light is announced on the other tank, and there should be no problems. It helps if they're watching their boss timers to anticipate the taunt coming, and not be on the GCD when it happens.