Bald Mountain - Velen
This quest automatically begins after you complete the Blood on the Battlefield quest. For the first part of this quest you'll need to go up the mountain and have a talk with the townsfolk here. After you go through a lot of dialogue with him, a little further up the mountain you'll bump into Johnny - and if you kicked Sarah out of the house during Novigrad Dreaming then she'll be here too.
Both of these guys will fill you in on who is on the mountain and how to get up there, head to the next quest market after speaking with them to meet the old lady and have more dialogue and some more scenes. You'll have to dive down into the water to collect the coin she drops (pictured below) to appeal her choice.
Use your Crossbow to defeat the Drowners underwater, grab the coin (use your Witcher Senses if you're having trouble locating it) and return to the old lady. She'll have you follow her assistant to a door which she unlocks for you. You'll meet Fugas, our next boss fight shortly after.
As for the Fugas strategy, much like most bosses you're going to want to close in for a few good hits and then back away again. Throughout this whole fight you'll have Ciri's help and she can't be killed, use that to your advantage. Fugas has a stomp ability which will stun Geralt and give him enough time to hit you with a Fireball, on a high difficulty this could be a real problem.
Fugas Weaknesses: Igni
There will be some scenes after beating Fugas - you'll be in control of Ciri when they're over. Follow the linear path to find The Crones and prepare yourself for a fight against them. Don't forget to use Ciri's special abilities (same button as Geralt's Sign) as often as you can. A tip is don't try to make the bubble very big - use it when all the enemies are grouped close around you.
<Strategy for crones>
On your way up to Imlerith you'll pass by a Place of Power that you'll want to draw from for the free Skill Point while we're here.
Imlerith is actually an easier fight than the Crones if you ask me. He has a pretty predictable mace attack that gets him locked in an animation for a few seconds making him vulnerable which is typically the best time to strike. He will occasionally teleport behind you to attack you - you can counter this by never standing in one location for too long.
When Imlerith glows you'll want to hit him with Igni, this will cause him to drop his shield. Aside from this my best piece of advice is to keep Quen up at all times, as far as bombs go Grapeshot and Dancing Star is what I would recommend you use against him.
Once you get him down to 1/3rd of his health he will throw his shield away and begin to rapidly teleport around the area - attacking you very quickly. Remember the five Ds of dodgeball and keep whacking him until his HP is 0. Defeating Imlerith will lead to some scenes after which the quest will end.
You'll receive the next story quest in the chain, Final Preparations immediately following this quest.