Disclaimer:
I own neither RWBY or Flash, unfortunately
Ozpin was having one of those days. No, not one of those days, one of THOSE days, when time slows to a crawl and nothing can speed it back up. So, he decided to go down to the armory/practice room. Oh, sure, he might hurt someone, or traumatize them in battle, or maybe, just maybe, he'd find a good opponent.
Ozpin was in luck.
Lie Ren was also having one of THOSE days. Jaune was out training with Pyrrha, Pyrrha was mentally screaming about Jaune's obliviousness, and Nora was out cold. Ren was actually an amazing fighter, and only Nora knew. Even she didn't know the full range of his powers. One gets good on the road with no home, or one dies. He held back so as not to be placed on a pedestal, a feeling Pyrrha knew all too well. So Ren went up to the armory/practice room.
Ozpin stared at Ren. Ren stared at Ozpin. They had agreed to a spar, with not their own weapons, but ALL of the armory weapons.
"BEGIN", shouted the virtual announcer.
At first they didn't seem to move, and then all the eye could see was two green blurs bouncing off of the walls and each other.
Teams RWBY, and JPN had gone to the library, and were studying when they heard the crashes. They ran outside, weapons drawn, and almost ran smack into Professer Goodwitch.
"Students... you'll want to see this."
16 hours later...
The entire school was awestruck as Ren and Ozpin still continued to fight. Parts of the building had long since collapsed, and new armies of fangirls had been created.
"A draw, then... Professer?" Asked Ren, who was wearing out.
"That... sounds good, Mister Ren" Panted Ozpin, himself tiring.
They clashed for a few more seconds, and then suddenly, they were still, in the center of the arena, shaking hands.
"You have considerable prowess, Ren."
"You certainly more than lived up to expectations, Professer. Say... How long has it been?"
There was a long pause, as the combatants realized that they had been fighting for over 16 hours, and that much of the female population of the academy, chiefly Team RWBY, Nora, and Professer Goodwitch were staring at them with streaks of blood running out of their noses. So of course Ozpin and Ren did the only thing that any sensible person would have done. They ran for Ozpin's office fast enough to give Oobleck a run for his money, creating a sonic boom as they went.