Lose a whole lot, learn the basic flow of the fight, try a well thought out strategy, lose horribly, rinse and repeat like four more times, finally get an idea that might
actually work, lose about fifteen or so more times, get lucky. That's how you solo a level 100 super Greninja with a Gallade.
If you were looking more for battle specifics though, I don't remember that much about the winning match exactly, but I can list my final move set.
1 Psycho Cut, 2 Swords Dance, 3 Double Team, 4 I believe was Agility but I'm not positive. I don't remember what my held item was either sadly.
I also remember that I needed to do only a very small amount of damage to it near the start of the fight, and to die as early into the fight as possible. (not a joke)