To strip the boss naked you need to destroy her clothes twice, until only the underwear is left, and then finish her with the Drive attack. After that in the defeat animation she'll strip completely.
The simplest possible way to do that is as follows.
It's even simpler if it's not the boss-only level.
Fight the boss using only the stunning attack. After initial strike sending the boss flying, next press on the button will teleport you to the boss, so just spam the button until the final attack sending the boss down to earth. One or two full combo is enough to destroy one clothes piece, but not enough damage to kill the boss. Each time you will destroy the clothes, you'll get the energy pot. When you get two pots, perform a Second Drive to get access to the Drive attack. Now the only thing you need is the third pot to perform the Drive attack on the close to death boss to strip her naked.
You have three options.
1. If the arena has barrels and one of the barrels contains the energy pot, that's all you need. Grab it, aim well and Drive the opposing Valkyrie to her defeat.
2. If there's no energy pots, but it's late game and you and the boss can do Connect, most possibly after stripping to the underwear she's already did Connect and became invulnerable. That's actually really useful to you, because you can continue to spam stunning attack, generating energy, without worrying that you accidentally kill the boss normally.
3. If you got to the boss in some Drive level already, you are all set, no need to worry at all.
If there's no energy pots in the arena, it's the boss-only level and you started the duel without Drive, you and the bosses didn't learn Connect yet, you will not be able to strip her, sorry about that. You will never reach the three energy pots required for that without killing the boss outright.