The Grand Melee

I have found a really easy way to do this myself, and thought I would share.

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• First you get 3 stacks of Defend like the others have mentioned.
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  Get directly behind the Valiant you are want to challenge and talk to it.
  
• The Valiant will start walking off the direction it was facing before you talked to it. Start pressing Charge immediately to hit it in the back! XD
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• The instant you Charge start pressing Shield-Breaker. You can hit them with this ability almost instantly, since your mount will be running a quick circle to face your target. They should now only have 0 shields up right at the start of the fight before they even touch you!
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• Now just stand on top of of the Valiant and keep using the Thrust ability, and keep your Defend stacked to 3.
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• When the Valiant starts to walk away start spamming Charge followed up with the Shield-Breaker like mentioned above. You should be able to kill the Valiant within 30-45 seconds this way.

Also if you are going for the Lance a Lot achievement, you can get it on the first day as long as you don't turn in the quest.

I def confirm that these fights are crappy designed... The game cheats lots to the point where you have to have 0 latency to stand a chance... You start the fight and they have 0 shields up, then instantly they have 2, and despite them being out of range for my shield break, they can still hit me. I'll have 3 shields up, they'll hit me once and I go down to 2, then before my shield cooldown is up they hit me again and I have 0, and then they'll run around far outside the ring and then charge me before I can get my mount facing the right way and go after them. I'll hit my shield break and get

So it appears that the secret to winning this is to forget every preconception of jousting.

1. Start with 3 shields, naturally.
2. When your opponent accepts your challenge, he'll attempt to ride out into position. Don't let him. Immediately start beating him about the head and shoulders with your lance. (Thrust)
3. When he tries to get to range to charge, throw a spear at his back. (Shieldbreaker)
4. Immediately follow that up by closing with him and beating on him again. (More Thrust.)
5. Hurray for honorable combat!

Be sure to keep your shields up. If you're a bit slow on closing, your opponent will get off a charge; try to stay on top of him. The best time to refresh your shields is right after you close with him. Either he'll get lucky and tag you as you close, in which case you need the refresh, or it'll give your shields time to recharge before he pulls away for another charge.