I discovered this by pure luck and after experimenting a little bit I think I understand why this trick works. This boss casts gigaflare to transition between phases, to skip phase 2 (or more accurately, avoid entering phase 2) you can get Bahamut prime's hp close enough to the 76% barrier, then wait for the boss to cast Megaflare (In this run, that waiting time was me activating Kaiten and casting Midare Setsugekka at 0:25). That way to transition between phases the boss has to finish casting megaflare, start casting gigaflare and then finish casting gigaflare. With some lucky crits, that's enough time to kill the boss without entering phase 2.