A flawless victory requires you to win all tourney challenges in "The Warble of a Smitten Knight Quest". This quest will automatically become available fairly early in the DLC, after rescuing the young knight in the arena.
This achievement is missable; you can complete the quest without winning all of the tourney challenges (the below details only concern the achievement, not the rest of this particular quest). You can practice the various parts prior to entering the tournament.
Specifically, to get the achievement, once you enter the tourney, you have to win (reload a save point before each one if you fail to win):
1. An "archery" contest. This is easy...just hit 9 targets with your crossbow before your opponent.
2. A horse race. This is a little more challenging. You need to successfully hit some of the track targets with your sword or crossbow (I found the crossbow targets way easier than the sword dummies) in order to have enough time to complete the circuit.
3. A grand melee, 5 on 5. Your team has to wipe out the other 5 knights before they get all of you. Not a tough battle, although you can't use magic.
4. A one-on-one vs. the previous year's champion Gregor the Mountain Gregore the something-or-other. This is actually pretty easy; although your opponent is huge, he's slow and you can just dodge/roll and hit him more or less at will.
If you win all of these, you'll unlock the achievement. Note that there is an option for quitting the tourney between the 2nd and 3rd of these. If you do so, you won't be able to get the achievement.