Transformers: Revenge of the Fallen

Whilst playing the Decepticon campaign, defeat Optimus Prime during the East Coast- Past his Prime mission.

Thankfully this mission is much more straightforward than it's Autobots counterpart. The mission starts with you having to defeat a few waves of relatively weak enemies and once done a brief cut scene introduces Optimus Prime. Pound him with headshots, use Overdrive if available and he'll be defeated in no time.