Anyone wanna take it in turns farming this with me? As I can’t find anyone online. Gamer tag: dale211191
Current level 207
Soul memory 7.1mil

MLGS nerf also affected this, not just CMW... my disappointment is immeasurable

if anyone wants help me to farm this, im lvl 200 soul memory 6.6mill
im playing on pc
Steam link: https://steamcommunity.com/id/colorcolorido

I farmed this with Mad Warrior at ~7M soul memory, Lv 220. I used the Unveil method, which some people in the comments incorrectly said doesn't work. It does, but you have to first clear out all the enemies until they don't respawn.

Equip the Bell Keeper's Seal ring.
Get Cleric's Sacred Chime and Unveil (Straid).
Get 13 Faith (alternatively, get 10 Faith and spice Unveil down from Magerold).
Clear out all enemies between the Ironhearth Hall bonfire and the Belfry Sol approach bonfire. This includes the 3 Alonne knights and 1 Ironclad Soldier on the bridge, as well as the 3 Ironclad, 3 Alonne archers, and 3 Alonne knights in the lava pit. After each, hit the bonfire and repeat the process until they stop respawning.
Then from the Belfry Sol Approach bonfire, run the gauntlet and kill the 5 bellkeepers until they stop respawning. You may encounter the Mad Warrior on these runs.
Once there are no more enemies to kill, hit the Belfry Sol Approach bonfire, and put the ladder just to your right (about 1 o'clock as you're looking up). Cast Unveil. If there is no Mad Warrior the Unveil will just go straight up (as it does when there are no enemies present), so hit the bonfire and try again. If the Mad Warrior spawned, Unveil will veer right through the grate/cage behind the ladder. Go and kill him, then Homeward Bone/feather back, hit the bonfire, and repeat.

With this process, I spent a couple hours clearing the room of respawning enemies, then got Hidden Weapon in 30 minutes. Godspeed.

Guaranteed to work, tested on my own Hidden Weapon sorcery achievement grind in Nov 2020. Uses the Unveil method with Mad Warrior.
You'll spend a few hours laying the groundwork (clearing the area of respawns) but it will pay off in the end when you're actually fighting the Mad Warrior. I got him spawned about 12 times in 30 minutes doing this.
There are comments down here saying the Unveil method doesn't work--that isn't correct. I suspect those people did not clear the area fully; I got ZERO false positives doing it this way. But it does require clearing the entire lava pit area first, which will take up the bulk of your time.

0. It may be a good idea to have the Bell Keeper's Seal ring on this whole time so you can get summoned to protect the bell--if you get a kill there it will count toward the total.
1. Buy Unveil from Straid in Lost Bastile for 2200 souls.
2. Get a Cleric's Sacred Chime.
3. If you're not already a faith build, get to 13 faith so you can use the chime and Unveil, or get to 10 faith and spice Unveil down to 10.
4. At the Iron Keep "Ironhearth Hall" bonfire, kill the 3 Alonne knights and Ironclad Soldier on the bridge, then hit the bonfire. Repeat until they no longer spawn.
5. Now do the same process for the lava pit area. There are 3 Ironclad Soldiers, 3 Alonne archers, and 2 or 3 Alonne knights if I recall correctly. Kill everything in this room until it doesn't respawn.
6. Now go to the Belfry Sol Approach bonfire. Run the gauntlet until the 5 Bell Keeper gnomes stop respawning--you may encounter the Mad Warrior during these runs.
7. Now that everything is dead and not respawning, hit the Belfry Sol Approach bonfire, and with the ladder to your right (about 1 o'clock) cast Unveil. If it goes straight up, hit the bonfire and repeat the process.
8. When Unveil goes to the right through the grate/cage instead of straight up, you've found a Mad Warrior. Go and kill him. Homeward Bone/Feather back, hit the bonfire, and repeat the process.

Anecdotally, if you also have the Delicate String equipped he seems to spawn more often, but I can't prove it.
If you need visual cues, check out this YT vid:

Without a doubt the most fun covenant item to farm for in any of the Souls games. I played DS3 first as I was late getting into the Souls series, and that game really did dumb down a lot of things and ruin others.

Any way to know how many times you have killed the mad warrior?

This game ****ing blows, forcing you to do PVP on a broken combat system for 100% completion.
To Obtain the most useless spell ever by chance of getting invaded and having 30 victories is enough to tell you how dark souls 2 is the worst in the series. **** You B Team

That is awesome

The one remaining sorcery to complete the achievement. I have never been so close to cheating.