Dark Souls 3: Lothric and Lorian

In the first phase, stick close to him, shield up (preferably the Black Knight Shield), and learn to dodge his attacks. There's a big window when he's finished, one-two hit and recharge your stamina. Watch out for his big walloping attacks, and when he teleports, he'll usually try to hit you right away.
It's broadly the same tactics for the second phase, but when they fire the little missile things, run around until they're all gone, do NOT ignore them, they will kill you. Remember that once he's down, he'll get up again with an AOE blast, so be careful.
Good luck!!