Follow Alyn Shir and she will lead you to Amethyn. Inside of Amethyn, you will only fight "Splinters of Fate" but each one is focuses on one style of fighting: Might, Finesse, or Sorcery. You will be able to tell based on their weapon. Usually the Might will use Great Swords and Hammers, Finesse will usually start with daggers but will sometimes have Fae blades, and Sorcery will have Chakrams. The only two things to really watch out for is that the Might Splinters can use the grappling attack and pull you towards them while the Sorcery Splinter can cast fast moving ranges spells at you.
They shouldn't pose too much of a threat as long as you keep on the offensive since they are easily stunned. Make your way through the dungeon collecting the Prismere and Prismere weapons scattered about the tunnels. When you start getting towards a small camp with what looks like a crimson throne at the end, you'll be close to the end. Just walk towards the Prismere wall at the back to start the final fight.
Tirnoch
Tirnoch herself has four or so different attacks. The first is a horn smash where she drags her horn across the front of the battleground. Whichever horn she lowers closest to the ground will be the one she uses to attack. You can simply back up to avoid this but be quick because it covers a lot more area than you think. She has an attack where she picks up giant shards of Prismere and throws them at you. You need to dodge at the last second to not get hit because these will correct quite a bit to hit you. She also has two roar attacks, one of which is a clear roar that she drags horizontally across the platform will the other is a fire breathe that deals massive damage no matter where you are. This attack means you need to keep your health topped off.
You must Fate Shift to be able to attack her. Once you do it, her face will fall to the ground and you can attack her freely. When you do enough damage, you'll move to a new platform and start again with new splinters of fate and no charge in your meter. It is possible to not do enough damage and you'll have to get another Fate Shift for the small amount of damage you didn't get this time so make sure you are hitting her.
When you move to the new platform, your meter will be drained again and you will fight more splinters of fate. On the second platform, you'll have to fight two Splinters at once. On the third and fourth platforms, you'll have to fight three at once. If you Fate Shift while there are splinters are out, they will all die and you still be able to attack Tirnoch as you normally would.
Knowing all that, there is a very, VERY easy solution for how to beat her. In Alchemy, you can create Potions of Fate using four Essence of Fate (you must have your Alchemy sufficiently high to make them). If you have four potions of fate, you can simply drink one after the first splinter drains your meter, then hit her until you move to the next platform. Wait for your meter to drain again, drink another Fate Potion, and attack her. Rinse and Repeat. I only had two and saved it for the last two platforms. This made it incredibly easy.
If you don't have the potions or don't want to go that route, you should now understand how the battle works. This is the last fight so feel free to use whatever potions you do have. Once you clear her health bar, you will have one last enemy to unravel. Make sure you get your +100% XP.
