Of the four bosses that prowl the Ringed City's dizzying heights and ashy valleys in Dark Souls 3's new DLC, Halflight, Spear of the Church is the easiest of the bunch. A little preparation, and completing one of the expansion's side quests, will help you make short work of the boss.
Find Lapp's Armor
One of the side quests in Dark Souls 3: The Ringed City asks you to help an NPC named Lapp recover his memory. Your reward for doing a good deed is a Titanite Slab and new set of armor. More importantly, you'll gain Lapp as an ally who you can summon to fight by your side when the time comes to face off against Halflight, Spear of the Church.
There are other, story-based reasons to give Lapp a helping hand. Without spoiling the outcome, fans of Dark Souls, Demon's Souls, and Bloodborne are sure to appreciate Lapp's connection to a fan-favorite NPC.
Enter the Boss Room
Make your way to the Ringed Inner Wall bonfire and proceed to the church. The doors should be closed. From within, a character's booming voice advises you to turn back. Totally ignore that advice open the doors. Inside, turn to the left and, provided you've completed Lapp's side quest, activate the summon sign to bring a certain NPC into the fight.
At the far end of the room you should see Judicator Argo, a giant and the character who warned you against entering. He gives you another long-winded warning, but that's fine: take advantage of his verbosity to buff your weapons, ready your spells, and equip any consumable items (flasks, stamina-regenerating herbs, and so on) you want to use in the coming battle. Once it starts, you won't have much time to fuss around in your inventory.
Fighting Halflight, Spear of the Church
Judicator Argo prefers not to dirty his hands fighting you. Instead, he opens by summoning a Painted Guardian—the same hooded enemy that guarded Anor Londo in the original Dark Souls. Make sure work of this foe; Argo summons the true boss into the arena shortly after calling in the Painted Guardian, and you don't want to give them the chance to overwhelm you.
What boss Argo summons depends on your network connectivity. If you're offline, Judicator Argo will bring in Halflight, Spear of the Church. If Dark Souls 3 detects an Internet connection, you'll have to face off against a player-controlled Spear of the Church, a call back to the Old Monk boss from 2009's Demon's Souls.
Halflight is tough, but not nearly so much as players, who are not limited by their AI routines. Go offline to increase your odds of winning this battle. Halflight moves and fights similarly to other human NPCs. That is to say, all you need to do to win is bait him into swinging his sword or casting his spells and then unleash your best offense.
Even when controlled by a human player, this boss has a relatively low pool of hit points. Stay on him and you should bring him down quickly, especially if you brought the NPC companion into the fray. As reward for vanquishing Halflight, you'll receive a Titanite Slab and a Filianore's Spear Ornament, a covenant item you can parlay to increase your rank in the Spears of the Church.