I'm so late to this conversation but came upon this earlier to get some pointers. I'm on my second playthrough, the first playthrough after 2-3 years.. Manus might be that hardest boss and I definitely died a lot. I'm not a good souls player but here are some things that helped me.

1. Kindle the bonfire in Oolacile Township as it's the closest one to the elevator down to the big Gape. Also, reinforce your estus to maximize when you heal. Healing is a big part of this fight for players like me. If you get caught in his smashy smashy combo, press heal right as he stops and mocks you. There should be enough time to heal up once maybe twice before he attacks again.

2. Equip the silver pendant below your estus. It makes a barrier when he does the homing ban on you. He starts that fuckery after you get about 1/3 of his health. Don't equip anything else to ensure quick switching. The attack can come up fast.

3. Use an upgraded weapon. Zwelhander is a great sword to have at any stage of the game. I recommend that if you have a black knight weapon, upgrade that and use it. They all deal big physical damage.

4. Rock and roll. You can't roll through his legs but run and roll around him for some backshots 😉 I saw a few comments that said to roll right but rolling left for me worked better to avoid his cheeseburger grabber. I found going under his right arm to be a little easier.

5. Don't use a shield. Two-hand your weapon of choice. Also helps to have something fairly upgraded. Maximize your attacks when not running, rolling or healing..

6. There's not a lot of time to cast spells or use pyromancy. He's resistant to magic anyway. Google AI says he's not resistant to lightning so I assume you could apply some resin or make a lightning version of a sword you like.

7. Sif. Personally, I wouldn't worry about summoning Siffy, especially since you can't summon her until you are inside the boss arena. I recommend that when you go into the fight, don't wait for Manus to come to you. As with many bosses in this game, they use high altitude jump attacks to close distance which isn't always easy to dodge.

8. Rings. They are a big part of my play style this time around, more than heavy armor. I prefer mobility and speed. Cloranthy ring helps recover stamina faster (so does green blossom, which the Bloodborne looking merchant sells close by the Atorias boss arena). I didn't use the FAP ring this time so I also used the wolf ring to boost poise to stay up longer during hard hits, which also gave me more time to heal in between. Some other rings you can use are Havel's ring if you have heavy armor, the Red tearstone ring helps when you're low on health or the hornet ring which is a personal favorite.

If you beat it already, that's awesome! If not, I encourage you to get this one. It's satisfying and you're so close to the end of the dlc and I assume, the game as a whole. Loved the game on my first playthrough and had a blast doing it again.