1. Go to the bottom left edge in Hidden Leaf
2. Use ledge attack
3. If you're lucky you'll get his animation where he jumps up and uses his sword.
4. Buut, since there is another platform above BM, he will keep trying to get to THAT bottom platform even though he is on the upper platform
In short, Black Mage spazzes out on the upper ledge trying to get to the bottom ledge.
5. Black Mage will keep doing this until he gets hit, be it a rasen shuriken or falcon punch.

I'm assuming Black mage might have a new animation for Beta, but I just wanted to let you guys know if you didn't.

wont he use his sword if his damage is more than 100 or something like that

This ^

Characters use alternate animations if their damage is high? Is it to do more damage?

It's called 100% ledge attack, only some characters can use it atm. Your HP has to be 100% or more if you want to use it.