i found new strategy to beating him 1st turn: use a light curtain to deflect his first mega flare 2nd turn: jump right when Buhamat's count down reaches 2, then you should completely avoid mega flare. Repeat turn 2 until he dies
No offense, but this is already a well-known strategy. Technically, you should enter the command on 3 so the Mega Flare actually hits and reflects back for damage. From Super Slash's Boss FAQ:
Since Kain is alone, make sure you put him in the back row when Dark Kain was there. Now, Lunar Bahamut will have a countdown timer. It goes from 3 to 0. When it reaches zero, he will use Mega Flare. This will most likely result in an instant death for Kain. So, when the timer reachs two seconds, jump. If you time it right, you'll miss the Mega Flare. However, he will counterattack you as soon as Kain comes down. For an alternate strategy, you could use a Lunar Curtain, so that the Mega Flare would reflect back onto Lunar Bahamut.
If you use a Lunar Curtain, be sure to jump at three seconds, so you can give him damage from the jump, plus from the Mega Flare. This battle will take awhile using the first strategy, so keep that up until you win.