Note: Lots of Witcher 3 achievements appear to be glitched/bugged for quite a lot of people. Often this can be fixed by restarting your Xbox and reloading a save. However, if this does not work then you may need to wait for a patch (Written 09/07/16).
This achievement is gained once you have won the tourney without losing a single event.
This can be done on any difficulty, however based on feedback I highly recommend
playing on the easiest difficulty if you just want this achievement. For advice on doing this on Death March difficulty, see the bottom of the guide.
I also suggest you have a good saddle for the horse race. The game should autosave at the start of each event in the tourney.
To start this quest ("The Warble of a Smitten Knight") you must continue with the main quest until you fight the rock thing in the tourney arena. After that you will receive this side quest to talk to the wounded knight. Talk to him, then save the game. Go and enter the tourney. There will be a few events to complete with some story tasks in between:
Shooting Range - You must shoot the red targets (I went as Geralt, and had to shoot these). Note that I shot two of the wrong targets before realising which I had to shoot and I still won easily.
Horse Race - You have to ride around a long track in a limited time. You cannot do this without getting extra time. To get extra time attack dummies and targets that are all throughout the course. The dummies are beheaded with a melee attack and the targets are above the course and must be shot with your crossbow. I suggest you have the perk "Lightning Reflexes" to slow time during aim.
Here there will be a cutscene and depending on which armour you choose at the start you will have either a fistfight or a mounted duel. You do not need to win this, it appears that only the four 'official' events count, however winning may count as the honour virtue for another achievement.
Make a manual save while following the bird since the cutscenes after the 5v5 override autosaves from before it (particularly if you are on a hard difficulty). After following the bird make sure to say that you will remain in the tourney.
Fight! - You must win a 5v5 by defeating all of the enemies without you falling (you cannot use signs, and your allies can be slain).
Duel - You must 1v1 a large knight who is pretty slow and easy to dodge.
After the duel your achievement should pop.
Death March advice, this is taken directly and unedited from the comments of
Kerman7, so a big thanks and all credit for this to him (expand spoiler):
*** Spoiler - click to reveal ***For those who want to do it on Death March, I'd say do make the save before going back to Guillaume and leave the quest there for a while, go level up doing other quests. I was slayed like a piece of paper at level 42 (recommended is 37 I think) but came back at lvl 47.
Also you can use potions during the 5v5 (1v5 after 10s really) but can't access inventory during it, so make sure you prep correctly, as you do. I'd recommend using Superior Hang Mans oil and equipping White Raffard, Thunderbolt, Swallow and White Honey (all superior if possible). Also go to the villa and sleep in your bed so you can enjoy more vitality and extra potions (you need to have upgraded the villa first), and beef up blade and armour too. Finally, don your best armour, you do not have to fight with the one you've been given at the start; I swapped my beloved Viper for a full Toussaint heavy armour just for this fight.
With all this, I actually only had to use I think two Thunder, three Raffard and one Honey (and lots of dodging), so probably can be done at slightly lower level.
The next fight against the single slow knight will start without a chance for a manual save, however all potions will be restored (plus he's a piece of cake).
A big thanks to ThePlatinumKush for all his efforts and providing information that I never got during my playthrough.
Also thanks to Kerman7 for his death march comment.