The Ripper Legendary Weapon can be purchased for $34,125 from the vending machine in Grace's basement. But, the Sawblade Launcher is only awarded to you for completing the DLC story.

With both weapons, head to an area with lots of zombies, like the Main Stage and start swinging with the Ripper. You need 10 hits, counters, attacks, etc in quick succession to supercharge the ripper. You will notice sparks flying from it when supercharged. Now kill five zombies with the supercharged Ripper.

Then switch to the Sawblade Launcher and instead of rapid firing, hold until the reticle becomes a large circle. Aim at a zombie's midsection and shoot to cut it in half. Do this five times!