UFC 2009 Undisputed

You cannot strike an opponent on the ground once you have them on the ground. Simply grab a good grappler (toggle stats with in the select fighter menu) and body slam them until you get them weak. Rotate your stick and click it a few times to get into a good submission move and rotate to break their body parts. Should unlock after he submits.

If your going for "Trophy Collector" you'll most likely be able to get this using the strategy there.