Everybody just keeps posting 6-8 for red, someone correct it finally, it is wrong. Fastest red monster level is 4-1.
6-8 is 4 waves, red and blue enemies and BLUE Boss.
4-1 is 4 waves, red and dark enemies and RED Boss.
11-6 is OK and then 20-4 you can delete 19-4 and 19-6, no need for that.