Completed all Time Trials achievement in Tomb Raider: Legend

There's a glitch that will help you with this.
Go to Replay Level and pick your level and set it to Time Trial. Then start the level, pause it, and select Quit, then select the level again and press Continue.

The game should have set the mode to whichever difficulty you beat the level on normally, but after you die for the first time in a level, you'll enter Time Trial mode again, respawning at your last checkpoint! So just get half way through a level, die, and you should easily beat the time.

This glitch was patched though, if you've already downloaded the update, then I recommend doing the Bolivia Redux time trial which will get you One Shot Kill - this even works on bosses and cuts combat time tremendously.