Enter these codes in the pause menu while playing. A note about the skills/upgrades/soul cheat: If you use that, you won't be able to get the trophies associated with the skill upgrades. Use it at your own risk.

Effect	Code
All skills, upgrades, and one million souls	Up, Down, Left, Right, Down, Up, Down, Up
Level skip	Left, Left, Left, Right, Down, Up, Down, Right