On the act: "the Great Wall of Vengeance," you will be strapped to a rocket. While on this rocket you will have to kill 100 enemies. This achievement is incredibly easy. It is about a 5 minute ride and there will be tons of indians on the ground and swarms of samurais riding rockets in the air. The samurais are easy to kill if you just find a place the first one of the group is heading as all of the ones behind him go to the exact location. If you do decent you will probably unlock the achievement well before you complete the rocket ride.