Optional: Midir
Maximum souls: 1,115,370
Before you are allowed to fight Midir in its boss arena, you must first knock it off the bridge near the Shared Grave bonfire. See the following video for a strategy to help you knock Midir off the bridge.
Midir on bridge NG+32 Dragonslayer's Axe, Dark Souls III
Midir is intimidating because of its size and enormous HP pool. The boss is easier to fight solo because it would not run/fly around the arena too much. Furthermore, Midir is easy to read as you constantly position yourself in front of it. Once you have fallen down the hole and landed on the boss arena, cast Sacred Oath and Deep Protection and use Yorshka's Chime+10 to buff a Dragonslayer's Axe+10 with Lightning Blade. Equip Lloyd's Sword Ring and/or Lightning Clutch Ring for extra damage output. Refer to the following for strategies on fighting Midir:
Midir NG+32 Dragonslayer's Axe, Dark Souls III
Dark Souls 3 Ringed City - SL1 NG+7 Darkeater Midir No Rolling or Blocking
Midir is vulnerable against Pestilent Mist. Use this spell if you want.