Here's my video guide:
BUILD
Partner: Jack
Weapons: Argent Wolf Blade (Fortification), Iceblood (Fortification)
Blood Veil: Noble Silver (Alleviation)
A. Blood Code: Ishtar
Passives: Swift Destruction, Strength/Dexterity Up, Two-handed Sword Mastery, Revenant's Ambition
Active: Supernatural Blood, Blood Weapon, Flame Weapon, Lightning Weapon, Adrenaline, Bridge to Glory, Overdrive, Hasten
B. Blood Code: Queenslayer
Passives: Swift Destruction, Mind/Willpower Up, Two-handed Sword Mastery, Revenant's Ambition
Active: Supernatural Blood, Adrenaline, Bridge to Glory, Hasten, Final Journey, Flame Protection, Fatal Surge, Dragon Lunge
NOTES
If you are missing some of the gear, that's ok. Give it a shot and tweak as needed. If you are still struggling, then do another playthrough and get what you need (assuming you still need to do other playthroughs for other endings).
Level 1 - if you drop down in right spot, you'll 1-hit kill the enemy. Easy!
Level 2 - the unarmed enemies should be priority as they attack faster than the one's with swords. But either way, it's important and easy, to dodge attacked. When the mini-boss comes out, focus on him above all else before he fully spawns in.
Level 3 - drop down, get the hits you need to build up your icor to do dragon lunge. back off and be outside the blast radius of the poison attack but line up directly facing the boss. Take a step forward and do dragon lunge, this should force the boss to power up. Just keep swinging and it should go down before it gets its next attack off. If it looks like you won't kill it before that next attack, just dodge and go back in with another dragon lunge.
Level 4 - have iceblood equipped. Dodge the initial attack and backstab if you can but if not, just keep wailing until it goes down. You may need to dodge another attack but with time, you'll get the hang of it.
Level 5 - you should be able to knock out 1 enemy before the other 2 attack. Avoid the sword enemies for last since they attack the fastest. When the mini-boss spawns in, focus on it and ideally it'll die before it can even finish spawning or do it's first attack.
Level 6 - the hardest part of this is timing the dodging. There are 2 different first attacks it'll do: throwing flame projectiles or lunge attack with follow up swing hit. Ideally it does the latter as it's easier to dodge (with practice). If flame projectiles, just try to dodge and towards the boss as quickly as possible. Get in enough hits to do dragon lunge. Back off as the boss powers up, finish off with iceblood.
CREDIT
My strategy and build was partially inspired by Noco:
Enjoy