In this video guide I will show you how to defeat the Skull King in the Gaol of Stagnant Blood. Now this fight will be a real pain for any level i'm sure. This boss is the king of one shots. The main things I used were the
Assassin's Sickle, Foulblood Barrier, Regenerator, Cleansing Light, and Elemental Wall. The main thing is to try to learn his moves first, to time the rhythm of what i'm about to tell you. Once you get his pattern, try to use the hold Y ability with the Assassin's Sickle to get a hit and run tactic. This also grants some ichor back as well. If you can time this like I do without taking too much damage, you and your partner basically be stunning him, not giving him much chance to attack. I used Yakumo for this fight for the extra damage he deals helps. Make sure the buffs I mentioned stay active as much as you can when you have the ichor. Any extra to spare can be used for projectile spells for extra damage.
All of his moves are pretty close to one shot kills. So any damage negating spells are a must to survive. He has a flurry of combos that have a real nasty reach. Even if the blade won't reach you, the aura will still hurt you. Once you damage him enough, from time to time he will have a whirlwind effect around him, causing only slight damage. This move is a slow build up so once you see it start, back away. He also has some jump-lunge attacks to close gaps between you. Can be pretty lethal if made by direct contact. He will also cast projectiles out from his swords, which can be up to a few hits of those. There is also a charge attack he has with his blood veil, can be pretty lethal.
Once he gets half way dead he will buff himself and you will tell when he is glowing. Now his damage will be amplified and the one shots will start to happen. You want to make sure at this point that the Fouldblood Barrier is active as much as you can. Get a few hits in if you dare and get out. Cast some projectile spells if you have to, to weaken him.
His ultimate move will kill you no matter what, unless you have Foulblood Barrier on or another skill that leaves you at 1 health. You will see it coming for a second, his swords will have seemed to grow in size. They will also be glowing red. The AOE of this move is enormous, it will basically hit you when you are standing way in the back. So be warned this is the move that will kill you the most. If you waste too much time he will use it more than once. So like most bosses, speed in killing the boss is important. Once he is defeated the achievement will pop.