Bone Marrow

Whilst the solution from Acheivement Land is a perfectly valid and suitable solution, another possibility for this achievement is also possible.

Instead of fighting the different enemies, and worrying about your/enemies health and damage output, simply collect items as normal until the first rat appears, once it appears DO NOT FIGHT. Instead move your character around the screen during the daytime state and collect all items on the outside edge of the screen, avoiding whichever path the rat is on.

The rat should be used due to it being unable to pick up items.

Doing this will allow you to farm as much resources as desired whilst slowly allowing your personal level to raise to 6 whilst the rat enemy is present. This is a slightly longer method however doing this will have you sitting with over 1000 defense stat (shield) and 5-600 attack stat (sword).

Once you've got the desired amount and have grinded to level 6, defeat the rat, wait for the 'boss' to appear and destroy them easily.

The King of the Dead is the last enemy you'll fight in the second area called Field
He'll show up when you reach level 6

Achievement Walkthrough: