Noob Saibot is accessible after you’ve won a Mortal Kombat tournament. You’ll have the opportunity to choose your reward at the end of the final match. Choosing the fourth option from the left - it looks like a backwards "3" - will open the match with Noob Saibot. Be aware that, should you lose, you will not have the opportunity to continue the match and try again. Instead, you’ll have to beat another entire tournament to get to him again. Fortunately, a loophole and a glitch dramatically simplify this achievement.
The loophole, discovered by mach II, is that you don’t have to beat the single-player tournament every time that you want to fight Noob Saibot. If you have a second controller, you can simply set up an eight-player tournament with the two controllers and proceed through the tournament fighting against yourself. After a few minutes, you’ll win the tournament and be able to select the fight with Noob Saibot as your reward.
If you try to beat Noob Saibot using conventional strategies and techniques, you may find him to be quite frustrating. He’s difficult to see and has a series of very powerful moves and counters. Unfortunately for him, DisturbedDan discovered the following glitch. If you simply press as fast as you can, Noob will get stuck in an animation glitch, allowing you to kill him in a few seconds and preventing you from taking any damage. Many people have reported being able to get back-to-back Flawless victories using this technique. Below is a video demonstrating this strategy.