Take the sai and use the highest difficulty. Start the first level. As soon the first enemies spawn, run further into the room and ignore the enemies that initially run towards you. This will force more enemies to spawn. Run to the back of the large lobby room to fight enemies that spawned earlier. There will still be a steady supply of enemies spawning. While building combos, go towards the section that connects the lobby you are in, to the other big room. When your enemies start declining, there will be another two on the way that help you reach a 300x-combo.