Vordt of the Boreal Valley guards the passage at the opposite side of the Cathedral where the priestess is located. It will be the boss at the end of the High Wall of Lothric area. Recommended level 20
 

• Weakness: Fire and Dark
• Drops: 2.000 souls and Soul of Vordt of Boreal Valley

Just like Iudex Gundyr, this boss is all about dodge and counter. Stay close to him and try to hit his legs whenever he starts his attack patterns. If you're quick enough there's also the possibility to get under him and do consecutive slashes on him. After half of his health gauge is deplated he'll get way more agressive and do extra attacks. He will charge towards you three times in a row, so time your dodges to stay away from him. Right after that, he'll start charging an attack. You have two choices: you can retreat to safety to let him release a breath of ice, or you can run towards him and hit him a couple of times, which will stop the charging of his attack and will give you an easy opening. Keep attacking his legs and looking for his back and he'll go down.

For a visual reference on the boss check the video below 

Vordt of the Boreal Valley Video Guide