Dark Souls III

Dragon Slayer Armour

The Dragon Slayer Returns 

Tales were told of the Knights of Gwyn. They were powerful, noble and fiercely loyal to their king. Now an imitation guards the way to the Grand Archives. A wretched reminder of an age gone by and a knight loved by many. What stands before the Unkindled is a hollow imitation of the once proud knight Ornstein. Do him justice and release this paragon from the control of the pilgrim butterfly. 

The Dragon slayer armour is tough, he can tank hits like a badass and has great resistance to lightning, not that you'd be using it because he basically exudes it. Help here is almost recommended I'd say since the Dragon Slayer Armour is incredibly strong, has high poise and can really punish an over aggressive player. The key is finding moments to pull back.

Eygon and Sirris really help during this fight and seeing as I needed them anyway for side quest reasons. With the Dragon Slayer down the way to the grand archives is open.

Side note: Eygon as a character isn't particularly interesting as he mainly sticks around to make sure that you don't hurt Irina of Carim. This makes his appearances in boss fights odd. When you first find Irina you take on the responsibility of looking after her, Eygon originally brought her to Lothric to become a fire keeper but as we know the job is taken already. He stays to keep an eye on her making sure you don't harm her, thats his role. They're a package deal, take one get the other. But as to why he should help you or even does is left up to speculation as he doesn't seem fond of undead based on your first conversation. 

Sirris helps you regardless as long as you don't join Rosaria's finger covenant. She is more interesting as she has her own agenda which you can help her with. Again for more lore I would like elsewhere.