Hey! LUMEN is the system we use for the mechanics of Clean-Up Crew. You can find the System Reference Document (SRD) by Spencer Sample here.
LUMEN is created with the idea of simple rules and fast paced combat in mind. The idea is to bring the power fantasy style of video games like Destiny to your table and it works like a charm. You can easily have combat with waves of monsters that lasts only 1 hours of real time, because it is so simple and quick. Most sessions I played have been between 1 and 1-1/2 hours, but if you want to turn your game into multiple sessions with an overreaching arc, you can easily do that as well.