Orphan of Kos in Bloodborne: The Old Hunters DLC has insane range, attacks that cover front and backside of his body, and has AOE effects. Once defeated, you will acquire the Kos Parasite as a reward.
After the battle starts, if you see the boss reaching for the sword, make sure to dodge a projectile coming towards you. You may also have to dodge a lunging attack as well as a 2-hit long range attack.
At close range, you need to beware of a quick 3-hit combo that covers the front and backside of Kos. You need to ensure that you do not get hit by the uppercut and dodge behind it at the third strike to open a window.
Once you get the boss to 50% of his total HP, he begins jumping around the entire arena executing an array of attacks. All these attacks are fairly easy to avoid, but you should be careful nonetheless.
After he reaches the ground, make sure that you are behind him in order to avoid damage from the explosion in the area. The best strategy in order to defeat Kos is to get behind him, deliver a couple of attacks in between his attacks and then dodging away.
Do not forget to share your own tips and strategies with us in the comments section below!