Oonar's Arm

I separated my informations for pulling the Sword and put them in the Sorrowbane comments.

Inside the Theater of Pain in Maldraxxus (not inside the dungeon) you'll find the Sword Sorrowbane stuck in the ground and [object=335655] attached to it. Both of them are clickable objects so try to grab them! Too weak? Then obviously you'll need to buff your strength!

Easiest Acquisition:

1. Buy a [item=181163] from [npc=169964] for 10 Gold or keep your [item=6948] off cooldown and bind it to the Innkeeper close to Theater of Pain (maybe check the location of [object=335655] inside the arena if you haven't seen it before)
2. 
   
3. Head to Glutharn's Decay at far east of Maldraxxus.
4. 
   
5. Click 4x [spell=327367]s, then instantly use your [item=181163] or hearth back to Theater of Pain.
6. 
   
7. Hurry up (80% slow) to get to [object=335655] within 2min (maybe you wanna use a Goblin Glider Kit on top of the stairs when using the Hearthstone).
8. 
   
9. Click [object=335655] to receive [item=181164]. Congrats!

TomTom:
/way #1536 51.42 48.47 Oonar's Arm
/way #1525 53.14 41.31 Teleport Scroll Vendor
/way #1536 53.63 47.93 Unusual Strength Potion
/way #1536 37 47 Battle Hardened
/way #1536 76.44 56.72 Edible Redcaps
/way #1525 51.16 78.75 Strength of Blood Vendor

/way #1536 59.73 73.02 Explosive Strength Vial

Buffs:

There are several buffs and their combinations that affect the text messages for pulling the Arm:

• [spell=306272] (10min, stacking up to 3x)
• 
  
• [spell=327367] (2min, stacking up to 4x)
• 
  
• [spell=333791] (2min)
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• [spell=334436] (25sec)
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• [spell=338385] (1min)

Sources of Buffs:

• Looks like [spell=306272] is only available when A Few Bumps Along the Way is active in Maldraxxus' "The Spearhead" area. You have to defend damaged artillery by defeating NPCs close to it (filling up a progress bar) and get this buff when a ballista's repair is finished. (The WQ is completed when 2 ballistas are repaired and I don't think it's possible to help on another repair, so not sure why it can stack up to 3x.)
  
  
• 
  
• [spell=327367]s are small mushroom objects you can click. You can have up to 4x stacks of this buff active but keep in mind that each stack also applies a 20% slow. The only ones I found are in Glutharn's Decay in the eastern part of Maldraxxus together with [spell=327352]. This shroom area is also the location of the Rare Spawn [npc=162711] (source of [item=181263] battle pet).
  I had to bind my Hearthstone to the Innkeeper near Theater of Pain or use [item=181163] to get to Sorrowbane within the 2min buff duration.
  
  
• 
  
• [spell=333791] is the effect of a clickable vial on a table in House of Plagues area in Maldraxxus. For combining buffs I don't think it's a valuable option. I am playing a Shaman and had to click the Redcaps, hearth back to Theater, fly to Plague Watch, click the vial and Astral Recall back to Theater. Even with the now doubled duration I can only think of Shaman, Vulpera (Make Camp/Return to Camp) or Warlock teleports being able to move that much within 2min, so not a match for an item you can carry until you want to use it. And there's another problem... you don't know if someone else clicked the vial and triggered the ~5min respawn timer.
  
  
• 
  
• [spell=334436] is the effect of [item=180771] that is sold for 1 Gold by [npc=166640] in Maldraxxus (outside of the eastern Theater of Pain walls). You'll need to do some quests close to his position to unlock the vendor option.
  
  
• 
  
• [spell=338385] is the effect of [item=182163]. You can get them from [npc=171808] in exchange for a specific amount of a Shadowlands crafting reagent (changing weekly) at the Night Market in Revendreth.

Progress seen when pulling and how to get there...

Spoiler

"You lack the strength to move it."

• without buffs

"You brace and pull, but are still too weak."

• 1x [spell=327367]
• 1x [spell=327367] + [spell=338385] combined
• [spell=333791]
• [spell=334436]

"You have a strong grip, but it did not move."

• 2x [spell=327367]
• 1x [spell=327367] + [spell=334436] combined
• 1x [spell=327367] + [spell=333791] combined

"The arm's grip on the sword is weakening."

• 3x [spell=327367]

Receive [item=181164]

• 4x [spell=327367]
• 3x [spell=327367] + [spell=334436] combined
• 3x [spell=327367] + [spell=306272] combined

List is not complete