How can I beat the mysterious figure with ventus?

There are many ways you can do this. First off you need a high level. Probably around level 70-80 depending on the difficulty.
You will also need to meld these abilities if you haven't done so.

-Second Chance| Cura + Cura + Pulsing
-Once More |Fira + Fira + Wellspring
-HP Boost |Fire + Fire + Soothing
-Combo F Boost |Cure + Cure + Pulsing
-Cure Boost |cure + Cure + Shimmering
-leaf bracer |Fire+Fire+Pulsing
-Damage Syphon | Fira + Fira Soothing
-Magic Haste x3 |Fire + Fire + Fleeting
-Attack Haste x3 |Slow + Slow + Fleeting





Each character has a special set of moves only they can learn for example.

Ventus can learn Salvation and Faith.

Faith does spread damage and can damage surrounding clones of the MF.
Salvation can do the same but can also heal you as well.

If you have these two then you can stand a chance.

But here is a list of the basic commands you will need to fight him.
-Dodge Roll at Max
-Super Glide at Max
-Any Shot Lock at Max
-Thunder Surge x5 at Max
-Curaga X3
-The Command finisher EXPLOSION (You NEED this)
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BATTLE
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This Guy has two attack Phases.
The first is a slow walk, like he is testing you.

DON'T LET HIM FOOL YOU!
Try and stay away from his as much as possible, for when he pulls off his long combo you can possibly get stunned and KO'd.

He can also do a sort of Lasso attack. Which pulls you up into the air and slams you to the ground.
Be careful of this attack, it can be devastating.
It's best to roll under him when he leaps high up into the air.

Another attack his a burst of wind which knocks you back and tosses your commands around the map. DO NOT attempt to fight him. Run around, grab your commands and heal. Leaf Bracer, Second Chance, and once more will all keep you alive if your HP is above 1.

There are several times when you can attack during this phase.
When:
A) He attacks you with that "X" attack.
B) After his combos
This is when you use Thunder surge. With a High Magic attack you can do some serious damage to this guy. But be warned not to attack him if puts up a guard like stance. He does have Cure Guard and will recover if you attack him like this.

Another attack he will do is clone himself, your in real danger if you get attacked here by even one of them. To get out of this use guard then your counter. This will build up your command gauge and let you use your finisher.

This is where Explosion comes in extremely handy. IT makes you invincible for a slight amount of time and can knock back the MF while doing a great deal of damage.
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His second phase happens when his HP drops below 3 bars.

This is where things get REAL tricky. He gains several attacks which include.
-Mega Flare
-Vanish

When he uses vanish hour lock on will disappear and you will no longer be able to see him. Use this opportunity to stay on the opposite side of the field, heal , and dodge roll spam away from him.

Just keep dodge rolling at this point and only attack when he fires that "X" at you. These are great opened up points and will guarantee you a safe fight.

Depending on your level you should defeat him pretty quickly and get your reward.. Hope this helps. ;D