For endless mode you will have to kill 1987 red dudes, you can do this across any endless levels, but I would recommend doing it on Bridge, Lobby, Office or Breaking as they are the fastest to grind kills.
There are 9 different levels called LAB#18, STAIRS, BRIDGE, BREAKIN, HALL, LOBBY, WAREHAUS, OFFICE, PWRPLANT each has 4 different modes. Endless, Race, Time and Real.
Endless: The mode you will be playing in, just killing red dudes.
Race: Get 30 kills as fast as possible.
Time: Kill as many red dudes as you can in 20 seconds of in gametime. (Think SPEEDRUN)
Real: Kill as many red dudes as you can in 60 seconds of real life time. (Think SPEEDRUNrt)
As you spawn in a different place each time, there are no directions that can be given. Just follow the video to that corner and wait there for the red dudes. Perfect spot for a katana.
This is the same map as 11BREAK. What you want to do it go in the hallway beside the glass room and wait around the corner, preferably with a katana.
This is the same map as 31LOBBY. Go to the right off the beginning and wait around the farthest corner (if at the main spawn). The red dudes will only ever come from the side you just came from. If the red dudes are not coming try stepping out and breaking the other pane of glass.
This is the same map as 27OFFICE. You will spawn in a different spot every time, so no use giving directions. But try to find the elevator and sit around the corner to the right. Get a katana as soon as possible.
(I know it says I dont have this achievement, but I have it preloaded)