So, while the purists will say "Permadeath or bust," I say: Use SaveMii weekly. Store a copy on your PC.
3 Minutes
Because in the real zombie apocalypse? You don't get a second save slot.
Your save file is corrupted. It’s dead. It’s a zombie.
Digital Resurrection: How to Escape the “Zombie U” Save File Nightmare
Change the hex value 01 (Dead) to 00 (Alive) at the offset 0x2A4 (Note: Offsets vary by update version—look for the isDead flag around the 600-700 byte mark).