I'm fairly certain you're referring to "Divided We Fall", in which the Cadmus storyline from the first season of JLU as contained on the DVD is wrapped up. The villian who receives the light-speed butt-kicking in question is Lex Luthor/Braniac.
Hope this helps.
> I'm trying to remember the name of the Justice League Unlimited episode where Flash saved the day by running faster than he ever had before (circling the globe in like a second), and then running into the villian. It's been awhile since I saw it, so I don't even remember who the villian was, I just remember it was the coolest Flash episode I had ever seen. He kicked butt! If anyone knows which episode I'm talking about, please let me know. Thanks!
That's exactly what I was looking for! Thanks!