Credit to ICE Faux Pirate.
Ripper
-Buy it from the vending machine in the basement of the safehouse where you meet Grace in Sola. Cost: $34,125
-Rev it up by smacking zombies (heavy attacks charge it faster). When it's spinning at max, slay 5 zombies with it. Doesn't need to be done during the same supercharged time. Takes 10 hits to charge it.
- "There is a circle with small lights on the ripper that light up as you hit zombies, all 10 light up when it's fully charged." -from user Mr SpinelesS
-If these kills aren't registering then charge it to max, then perfect time block and press X to kill zombies with Counter 5 times.
Sawblade Launcher
-Talk to Grace at the safe house after completing the DLC main questline to obtain it.
-Bisecting a zombie means to cut it in half at the torso. Lower a zombie's health with a melee weapon then get the last shot a little above waist level with the S Launcher. They won't always cut in half so you'll probably have to kill more than 5.
-You can also try charging it with 3 blades and aiming at their midriff. This seems to have worked pretty well for some people.
-You can buy the blueprint for the sawblade launcher ammo at the vending machine in Grace's safe house when you run out of ammo