If you had, say, a Level 1 Mr. Mime with Seismic Toss against any opponent with an HP of 100, then that would result in a 100HKO. The build would have to be:
Mr. [email protected] Leppa Berry
- Seismic Toss
That's all you need, really. Recycle lets you restore Seismic Toss' PP with the Leppa Berry so that you can keep whittling HP. You would also need a really weak opponent with no recovery or attacking moves.