A Flawless victory simply means that you take no damage over the course of the round. Keep in mind that you do take a small amount of damage when you block, so your strategy should be to avoid getting hit at all. While you can do this with whatever character youre most comfortable with, I recommend Smoke, Scorpion, or Sub-Zero. All three of these characters have moves that will allow you to immobilize your opponent repeatedly, letting you deal them damage without any risk of being hit.

Smoke: Trident Spear Back, Back, A
Scorpion: Spear Back, Back, A
Sub-Zero: Freeze - Down , Forward, A

Follow up any of these moves with a simple uppercut (Down,X) or the combo of your choice. As soon as your opponent recovers, use the move to immobilize them again and repeat.