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!

Having trouble with time trials? Try completing the final time trial in Bolivia against the final boss first (it's a boss fight, and it's easy). This unlocks the one-hit death cheat. You can input the cheat when the cinematics are playing at the beginning of a stage and make enemies and certain bosses die quickly, saving you some much needed time. It also makes the motorbike races faster to finish. Note that only one cheat may be turned on at a time.

Cheats appear in the Specials menu and the buttons are respective to your version of Tombraider Legend. Cheats are only unlocked when you clear the game or meet the following criteria.

Textureless Cheat - Clear the game (any difficulty).
Show enemy health - Clear Bolivia under 12:30 in Time Trials.
Infinite MG415 Ammo - Clear Peru under 21:30 in Time Trials.
Infinite RC650 Ammo - Clear Japan under 12:15 in Time Trials.
Infinite shotgun Ammo - Clear Ghana under 20:00 in Time Trials.
Infinite grenade launcher - Clear Kazakhstan under 27:10 in Time Trials.
Bulletproof Lara - Clear England under 27:00 in Time Trials.
One hit deaths - Clear Bolivia (stage 8) under 4:15 in Time Trials.
Unlock Excalibur - Clear Nepal under 13:40 in Time Trials.
Unlock the Soul Reaver - Clear the game and complete all time trials.