Here is a video guide, but for a written guide:
1. Beat the 3rd boss and go to Tohan Passage.
2. Go to the upper right to find a crypt.
3. Walk to the far right to find the Game Wizard Book.
4. Open your menu, go to accessibility, and you can find the cheat codes:
- NINPIOUS - Invincibility Cheat Code
- SEPPINOT - Infinite Jumps Cheat Code
5. Type one of these codes in, and the achievement is yours!
*This will not lock out achievements.*