The Lynx can be bought from the 3D printer. Get it before round 20.
Soon as round 20 begins you will be teleported to fight Big Zombie Oz. Use the Cure Station in the centre of the map to bring down his shields then lay into him as much as possible. You then have to run around the room turning on all the generators to use the Cure Station again.
Once his health is very low and you only need to hit him one more time switch to the Lynx and finish him off