Data-Axel is destroying me.

[Zephyr]

Personally I think this fight is extremely difficult if you don't use Limit Form. Last Arcanum and Sonic Rave grants temporary invincibility frames and it also heals your HP.

If you're not using Limit Form, Guard Break (might need Negative Combo) and Thundaga are really good at making him stunned.

 

[Sanmuri]

Data-Axel is the easiest - right after Vexen.
Unequip all Combo Pluses. But have Finishing Plus.
After Axel attacks: Reflega + Blizzaga + Blizzaga + Blizzaga. Remember to use Mysterious Abyss for Blizzard Boost.
When you've done Reflega + Blizzaga -combo enough, Axel will do his DM. Just Aerial Dodge and Dodge Roll.
When Axel's down to the last bar, use your Blizzaga-combo and then one more Blizzard to activate his Reaction Command.
And tada, done.

 

[Zephyr]

Data-Axel is the easiest - right after Vexen.
Unequip all Combo Pluses. But have Finishing Plus.
After Axel attacks: Reflega + Blizzaga + Blizzaga + Blizzaga. Remember to use Mysterious Abyss for Blizzard Boost.
When you've done Reflega + Blizzaga -combo enough, Axel will do his DM. Just Aerial Dodge and Dodge Roll.
When Axel's down to the last bar, use your Blizzaga-combo and then one more Blizzard to activate his Reaction Command.
And tada, done.

Thanks for the tip Sanmuri! I had used Blizzaga before but didn't think to use the finisher Blizzagas.