Pretty sure the "Megaflare" in the video was right at the end when loui died. It's the only time in the video that Bahamut "charges" his attack up like he always does when he does megaflare.. It is also a singular flare/eruption.

I think the attack you guys are talking about is similar to "Ultima" or something, as it reminds me of this -http://www.youtube.com/watch?v=7WQiuYVNt3s