Gehrman, the First Hunter, can be considered as one of the hardest bosses of the game, due to his array of fast attacks and quick dodges. Below is a description for each of his attacks.
Scythe Hook - Gehrman will extend his scythe towards the player trying to hook them with it, then slash from the other side of the player. It is possible to dodge his second attack with a correctly timed dodge. To dodge the entire attack dodge backwards once or twice depending on the players distance.
Charge Scythe Slash - Gehrman will charge for an attack with his transformed Burial Blade during this brief charge it is possible to stagger Gehrman. A reliable method for staggering Gehrman is to back up out of range and fire the pistol as he unleashes his first attack in order to interrupt the follow up slash.
Air Scythe Slash - Gehram will jump up into the air he will hover and begin charging his attack. The attack will send explosions along the ground that travel to the players position upon its release. Players can continue to dodge backwards to escape this attack. It is advised to be out in the open to prevent the player from being cornered against a wall during the attack. Also, it is practically impossible to attack Gehram during this attack.
Multi Slash - Gehrman will attack the player with an array of slashes from left to right. This attack will only be used when the Burial Blade is not in its transformed state. It is advised to dodge backwards and away from Gehrman during this attack.
Visceral Attack - If the player is charging an attack, Gehrman will attempt to stagger the player, followed by a Visceral Attack. This attack can deal massive damage. Gehrman can only do this while his Burial Blade is not in its transformed state.
Arcane AOE - Gehrman will stand still and extend his arms for 6 seconds then unleash a blast on the 7th second (approximately). During this time he is completely vulnerable so it is advised to get as many attacks in as possible. Be careful as this attack can instantly kill the player or severly injure and the player if hit should remove themselves from the battle to heal quickly.
Click to see dialogue
"Good hunter, you've done well. The night is near its end."
"Now I will show you mercy."
"You will die, forget the dream, and awake under the morning sun."
"You will be freed... ...from this terrible hunter's dream..."
Submit Your Life
"Farewell, my keen hunter."
"Fear the blood."
Refuse (or attacking Gehrman at any point)
"Dear, oh dear, what was it? The hunt, the blood, or the horrible dream?"
"Oh, it doesn't matter."
"It always comes down to the hunters' helper to clean up after these sorts of messes."
Gehrman, like any other Hunter enemy, can be riposted.
Gehrman's ranged scythe attack from the air, has incredible range, as such, running backwards or spam dodging is the best course of action.
When Gehrman buffs himself, he will not be stunlocked as easily. At this stage of the fight, Visceral attacks may be a better option.
If the player attacks Gehrman before speaking with him, the boss fight will commence immediately.
Killing Gehrman at the same moment the player dies still rewards the player with the Old Hunter Badge. This would allow the player to purchase Gehrman's Set and the Burial Blade in NG even without getting the true ending. Walking into the boss room afterwards would send the player to NG+. The same can be achieved if the player is killed by the Moon Presence boss that appears immediately after, if three Third Umbilical Cords have been consumed.
Due to the way there are white flowers in the location, and that the fight is versus a single person, the boss fight could be a reference to Metal Gear Solid 3 final boss fight, The Boss.
Gehrman wears a set that though unattainable to the player, is quite obviously based, if not just an alternate version of the Charred Hunter Set that is found in Old Yharnam, along with the Top Hat.