To unlock the Sand Nemesis achievement in Prince of Persia , you will have to kill at least 50 enemies in a row without letting anyone hit and without using the time dagger.
It is one of the toughest achievements in the game, but it can be done easily at “Rekem”.
When you reach Rekem, you will eventually be in a place where you will find three glowing sand creatures. These sand creatures have the ability to spawn other sand creatures.
You can easily kill these spawned sand creatures with a power slash.
Kill a few sand creatures, and run around until they spawn other sand creatures. Keep repeating it until you have unlocked The “Sand Nemesis” achievement.
Here is a video with some other achievements to unlock