1 Hp : 1000
The second Black Waltz is two times stronger than the the first. His HP is 1000.
2 Garnet won't be attacked
The second black Waltz will hardly target Dagger so don't worry about her and worry about the rest of the party.
3 Don't use Vivi's fire
If you used Vivi's fire then the Black Waltz will counter attack with his fire, causing a lot of damage to your party. Don't use Vivi's fire. In addition to that Vivi's magic won't affect him much.
4 Use Steiner's magic and Zindane's sword
Steiner's magic sword will be very effective against the Black Waltz and it won't result in a counter attack. Zidane's sword will also be effective.
5 Let Garnet cure all every turn
Let Garnet use the cure all ability whenever her turn comes just in case. If, however, one party member was badly hit and the rest were fine then it's OK to focus the cure on him.
6 Steal his items
The boss carries these two items: Steepled Hat/Leather Plate. Stealing them might be a good idea.
7 Don't let the men die
When the Black Waltz's HP goes below half he will target all males in the party with fire. While he won't attack Garnet you will still be in trouble if all men died. See how to defeat Black Waltz no3 - Cargo ship.