This achievement is quite difficult and requires patience. As with the original game, the main thing to watch out for with enemies is the awareness meter above their heads. When an enemy is alert, the lightning bolts above their head will begin to turn white and fill in. If the bolts turn red, the enemy has found you, and will begin to attack. If this happens, reload from your last save. White bolts will not end your achievement, but the enemy may run toward you on three white bolts. Hide until the enemy is no longer alert, and then continue. The sound of your footsteps while running or walking quickly may alert nearby enemies. To avoid this, move in Stealth Mode using as much as possible.