Potentially difficult, but far from impossible. Let's have a bash at the skeleton king.
Dark Souls 3 boss: how to beat High Lord Wolnir
High Lord Wolnir is a pretty manageable fight once you know what's going on - and provided your melee damage output is high enough. Ranged and magic users will have a tougher time of it. There are usually plenty of summon signs here, so take advantage if you're not built for melee.
The fight takes place in a tunnel-like arena, and High Lord Wolnir advances along it. If you wander to the sides of the area immediately in front of him, you may be insta-killed by the power of the Abyss, or just fall in a hole. Stick to the main path.
This is a battle you need to finish quickly. If High Lord Wolnir gets to close to the end of the arena, there's no way to dodge his attacks. You need as much melee damage as you can, so bring your best weapon and be ready to switch to a two-handed grip whenever you attack.
With those preliminary words of caution out of the way, here's the deal: you need to break the High Lord’s bracelets. He has two on the right wrist and one on the left wrist (relative to you, as you face him). It seems as if you're doing no damage as you slap away, but when a bracelet breaks it'll take off a third of the boss's health.
The general strategy is as follows: when the battle starts, let the boss drag itself forward and wait for its arms to stop moving. Most players favour the right arm to start; rush over to the outside edge of Wolnir's arm and paddle away with your weapon in two hands. When the arm starts to move, roll away to avoid a flurry of attacks. Rinse and repeat until both bracelets on the right arm are broken, then switch to the left wrist.
When High Lord Wolnir attacks, a good shield and some hoarded stamina will guard you against damage if you have trouble figuring out when and where to dodge. Do not lock on; it's actually quite hard to hit the bracelets if you do, and when Wolnir moves his arms your target will shift or the camera will become extremely unhelpful.
It’s trickier than it sounds, of course. You have to be careful of getting too close to the High Lord; if he manages to drag his rib cage onto you, you'll be instantly killed. If you attack his wrists from the inside edge, the movement of his arms may drag you towards him.
While all this is going on, the High Lord will attack you with slaps and sweeps of his arm, and after a short time, loads of skeletons and wheeled skeletons will appear. The High Lord's attacks usually clear these out, but be careful of them if they swarm as you're going for a bracelet. Part way through the battle the High Lord may summon a sword, which makes his attacks deadlier, and he has a magic attack - but it has a very long wind up so as long as you're not focused too narrowly on your target or squashed up against the wall, you'll easily avoid it.
The most dangerous thing to watch out for is Wolnir's vomit attack. The abyss matter he spews out does heavy damage over time. You can sometimes dodge the damage if you shelter in the lee of one of his hands, but it's best just to get as far away as possible. Wolnir's hands are still while he vomits so if you're happy to take some serious damage you can sprint in and out and hit the bracelets a few times during the attack, or use ranged and magic attacks to get a little bit of bonus damage on the bracelets.
You’ll receive the Soul of High Lord Wolnir as a reward for beating the skelly king. Activate the High Lord Wolnir bonfire.
Explore the bonfire room thoroughly to find the Grave Warden Pyromancy Tome. We can't use this yet; Cornyx won't accept it.
Be sure to go check up on all your pals at Firelink Shrine and cash in all your souls before moving on.