Play in Survival mode. Allow enemies to hit you until you have one or two hits remaining before death. This is necessary to extend the time spent in the "bonus" level after about 125 kills. Once you are this low in health, knock out all the enemies around without missing or taking damage to keep the streak going. The lightsaber bonus level will get faster over time, but it also speeds up if you are at full health. Being at lower health when you trigger the lightsaber level will allow it to continue longer before getting too fast and breaking your kill streak. If you did not break your combo earlier, you should have about 125 kills by this point and only need to survive five more "rounds" of the bonus level.