|You'll begin aboard your Yol racing alongside Cirina, Gosetsu, Lyse, Hien, a Mol Hunter and a Mol Shaman. As you reach the river you'll be attacked by several Budugan Warriors, Budugan Hunters, and Budugan Shamans. Fight them as best you can, but more will keep appearing as you fight. Eventually Gosetsu and your Shaman will stay behind while the rest go on, just follow the group instead of sticking with Gosetsu.
A short ways later they will be assaulted by Oroniri Hunters and Warriors. As before, just keep fighting until Lyse and your Hunter stay behind, then go with Hien and Cirina to the goal.
At the Ovoo you will find the Dotharli Hunters, Warriors, and Spiritcallers fighting the Oroniri and Budugan forces. Occasionally they'll attack you as well. Try to just stick to the units attacking you and avoid catching any extras with AoE attacks. Your end goal is to use the Ovoo and fill your meter, but you cannot do that so long as anyone is attacking you.
As you get close to finishing, Magnai will take notice and use Holmgang to pull you away. After a cutscene the fight will resume with Magnai on the field. He must be defeated to end the battle, so focus on him whenever you can. Cirina will find an opening and rush to the Ovoo, but Magnai will interrupt her with a Tomahawk. An Oroniri Warrior will then approach it, and you must defeat him before he can attune with it.
After this, Sadu Heavensflame will enter the battle. Like Magnai, you must defeat her in order to win. A Budugan Hunter will try to claim the Ovoo as well, so take them out first before resuming your attack on Magnai and Sadu. Both will use AoE moves on you every so often: Sadu has Dispelling Wind, a long line AoE, while Magnai has Violent Earth, a circular AoE that chases you.
Occasionally, Sadu will begin casting Falling Dusk. Two Stellar Chuluus will appear tethered to her, while she will be invincible. Both of the walls will need to fall to interrupt her attack, or the fight will be over. Magnai will actually assist you in taking these down, which is good as they can be quite sturdy.
As Magnai gets low on HP, he will use Broken Ridge, essentially the WAR LB3, buffing all Oroniri and Budugan forces with Vulnerability Down. Sadu will then focus on Magnai, and summon several Dotharli Chuluu around the edge of the arena. They'll all use Epigraph, criss-crossing the arena (and harming all Oroniri and Budugan they strike), so stand in a safe spot and wait it out. Focus on Sadu from this point on, as Sadu will be attacking Magnus and can eventually defeat him herself. Once both are down, take out any other forces attacking you, and claim the Ovoo. The Nadaam is now complete.
...however, the quest is not. After a lengthy cutscene Grynewaht will interrupt alongside several Doman Haplomachus, Doman Laquierius, Doman Signifer, and an Armored Weapon. You'll be joined by all your allies from the Nadaam, as well as the Oroniri, Budugan, and Dotharli. Lyse will also use Way of the Will early on to buff you and your allies with Damage Up and Vulnerability Down. Ignore the regular troops, more will keep spawning and your new allies will deal with them. Instead, despite the name you'll want to focus on the Armored Weapon first, as it will use Diffractive Laser to AoE your allies and isn't actually that durable.
Once the weapon is down, focus on Grynewaht. He mostly uses his abilities from earlier fights like Augmented Suffering. After knocking a bit of his HP down, Hien will ready Crush Weapon, and finish him off in a single blow (300000 damage). With just the remaining troops left, Magnai will use The One Ruler to fetter most of the enemies while Sadu readies Falling Dusk. Several more will assault the Stellar Chuluu, which must be defended until Sadu can finish her spell. Once she finishes the duty will be complete.