Unverified. If you want to do the Hard Mode without suffering too much, start a game on hard mode first. Finish the first stage (Bolivia), save the progress and quit.

Now load a completed easy (or normal) save file. Go to any mission and activate any cheats you unlocked. Once you have the codes active, go to the option to save rewards. Next, open the menu and choose to load your hard difficulty game.

The cheats should be active for your hard game; however, they will reset (default to off) if you quit. But you can repeat the process. Having the invulnerability and one-hit death cheats for hard mode will make the game much easier!