I would fast travel to Arden's Cottage (unless you have something closer) and start heading N. When you get to the House of Ballads, enter the House of Accolades. Inside, talk to Alyn Shir who will tell you to talk to Glianal. He will not help you so you'll have to talk to Alyn Shir again who will send you to talk to a being called Nyralim.
As you make your way to meet with Nyralim in the SE part of Dalentarth, you can actually stop by Didenhil too. Stop by the Three Lanterns Inn and talk to Agarth while you're there. Doing the Hunters Hunted quest is quick and painless while you're here. Skip ahead if you'd like to do that now and return when you are done.
You will have to make your way through Glendara and either Lorca-Rane or Haxhi to get to The Sidhe. Once there, you want to travel to the SW point of this region and enter Caer Nyralim. Inside, make your way along the winding path to the top of the tree and talk to Nyralim himself. He will say that he is uncertain of whether or not he should help you and send you on an errand to see what you do. He will send you W to Haxhi to deal with a tribe of trolls. When you get close, you will have to cross a bridge. Once you do, you'll have a fight on yours hands.
Troll Attack
You'll come across three sprites, two boggarts, and a Rock Troll. You will see a big tree stump in front of you too. You want to stay to the S of that tree stump because if you venture N, another Rock Troll will join the fight. If you do accidentally trigger the second Rock Troll, use the tree stump to keep them away from you while you prepare to attack. Once you have kill everything outside, you will have to trigger this Rock Troll anyway but it's a lot easier fight with just one Troll at a time. Head N and kill that Troll too. Enter the Dam that just opened up.
Inside Haxhi Dam, you'll bump into an old friend. After a chat, make your way into the Dam. The path is mostly straight but when you get to a room with waterfalls, you'll be attacked by a Troll. He'll be tough but you'll have help. While he's distracted, keep some distance and keep attacking. It seems that Trolls prioritize targets based on proximity so just be the furthest target and it should be easy.
Continue through this tunnel and just before it turns, you'll fight some Tuatha. Doing so shouldn't be particularly hard at this point, especially with Alyn to help you. Keep killing Tuatha until you come to a larger room with some Tuatha and a Troll. If you time it right, you should be able to execute a stealth kill on one of two passing Tuatha but they frequently cross paths so you need to time it well. With one Tuatha down, the fight will be a little bit easier. Take down the other Tuatha and then the Rock Troll.
Further along the cave, you'll come to a large encampment of Tuatha. They will be fast and aggressive. Range won't help you much here so you might want to get in close and use physical attacks and parries. Once they are dead, a Rock Troll will come through a wall to the N. Deal with this Troll like the rest and continue N.
You'll come to a room with a sleeping Troll and a dancing Tuatha near him. There will also be a Tuatha by a Sagecrafting station. You can stealth kill one of them but as soon as you do, Alyn will attack the other so you won't be able to kill both without a fight. Kill him too and hopefully you haven't startled the Rock Troll. Either way, kill them all and make sure you check out the treasure nook to the NW for some breakables and two chests. Take the tunnel SE from here and watch for a little treasure cache in a little nook on the right. Not long after that you'll come to the boss fight so be prepared.
Gnarsh
Gnarsh is tough. She only has two new tricks but she does have a lot more health and hits a lot harder than any of the other Rock Trolls. In fact, her charging attack, for me, was actually a one hit kill. Watch out for her double swipe attack. When she swipes with her club, she will always swipe a second time. This applies to the stationary overhead swing as well where she may also swipe to the side after. This is of particular importance because you may need to heal if you are too close to her. If she hits with the first swipe and you can't get out of the way of the second, you'll probably die if you don't heal. However, you can use this to your advantage because if you can get out of the way for the first attack, you may be free to lay on some hurt during the second.
Also of note in this fight are two environmental factors. Alyn is completely useless and occasionally, crabs will come out and get in your way. You actually can't run through these harmless, useless little critters so watch out for them so you don't suddenly die when you should have just simply dodged.
She should fall and you'll get some good loot, possibly some Essence of Fate. Travel back to Nyralim and see what he has to say about this. He will now allow you into Ysa so from the Caer, head NE. You have to cross a small pond but there will be a path. Enter through the vines to get to Ysa.
