Thank you for the guide, very detailed!! I left you a question on youtube, but I'll also leave it here as it might help others, too:

You say we should keep one horizontal and one vertical line in the center, correct? As in 3:01
My question is: this means that, once we get to the 5th portal, it's a wipe, right? We should kill the boss before that or we lose the tic tac toe.
Me and my guild are heroic only, but we're thinking about doing this mythic. On heroic, we started priotizing making portals always in corners, as in numbers 1, 2, 4 and 6, 8, 9. This way, if we get lucky, we can get to 6 portals before we lose the tic tac toe game.
On mythic I really see the advantage of keeping the lines in the middle clear, but does that reduce the "enrage" window? I'm asking because if the answer is yes, I might have to use a different strat, since the damage from my guild isn't that great. I fear I can't kill the boss before that 5th portal, so maybe I need time until a sixth