Got into the twilight realm right after he hits you in melee so you go in and when you get back (and aunt) you're in time to take the next melee swing.
I love how this myth that heroic will is what drops your threat still persists . It's fading light ending, "waiting til the last possible second" doesn't reduce the chances of him meleeing the wrong person.
Unless it lets your computer phase you back in quicker to land the taunt.
We killed it on 25 man heroic a few weeks back with when tank when one of our tanks derped and thought he was supposed to be dpsing, it was by no means clean and i wouldn't recommend it but we outgear it enough that the loss of a few dps from Ultrax crits still allowed us to kill it. I'm sure the effect would be greater on 10 man though.
He means if you just press heroic will you don't drop threat when you re-enter the fight in twilight realm, you have to of had the fading light debuff and then go into the normal realm via heroic will press to lose aggro upon return, this was so that the tanks 'had' to switch so the 100% dmg buff from fading light debuff after it fades didn't 1 shot your tank. However when 1 tanking you taunt the boss back instantly and resume, taking the 100% dmg increase hit regardless.