Zero the Kamikaze Squirrel Cheats

At any point during the gameplay press:

In the second level of the first area, kill all of the enemies in the level. Since there is no boss nor any level-altering tasks, backtrack to just before the second checkpoint. There, inflate the door that appears because all of the enemies are destroyed. Go into the door and get through the mini-game. At the end, you'll see one of Zero's family members and he'll tell you that his spear (the Zodiac Spear) is stronger than Zero's equipped weapon. You obtain the spear afterwards, but you can only equip it by entering the "equip weapon" cheat at the start of the game. The Zodiac Spear deals much more damage than Zero's original weapon, and it'll at least kill all normal enemies in one hit.