Muddle 5 of the grapes in a mixing glass. Pour the pineapple juice, lime juice, St. Germain, pisco, and simple syrup into the glass. Add ice to the mixing glass, shake, and then mix in the Champagne. Strain the contents into a chilled martini glass. Slide the remaining 3 grapes and lime mint onto a long pick, and put the across the top of the glass. Add liquid nitrogen to garnish the cocktail.