Step 1: Get the Ephemeral Enhancement Gobble gum
Step 2: Buy the RK5
Step 3: Go up to the Shieva wall buy, Buy the Shieva, wait a split second then hit up on the D-PAD and double tap Triangle/Y
Step 4: Press up on the D-Pad, if nothing happens, the glitch worked
Note: DO NOT SWITCH WEAPONS AT ALL ONCE THIS STEP IS COMPLETE.
Step 5: Build the zombie shield
Step 6: Pick up a Death Machine drop
Step 7: Wait for the timer to get to 0 and you will have the unlimited death machine.