The Horde Slayer

(this comment was originally written for the Horde version of this achievement)

My group and myself achieved this Feat of Strength on a number of different characters on 08-14-2018 on patch 8.0.1.

My advice to you all is to group up with a 5-man party (or larger, as noted later) and pick on small groups/solo players. It is not honourable, I understand this, but it is effective. The rest of this info and advice is based around having at least a couple other like-minded people in your crew, including at least 1 healer and as many high-mobility classes as you can convince to come.

Our process:

1. 
   
2. Enable War Mode. Take the boat to your opposing faction's island. Patrol your chosen zone (we used Drustvar as Horde players).
3. 
   
4. Kill people (duh) - be mean about it, depending on the quality of the players you are facing, you may be able to farm them for extra kills. We were not able to determine what gets you a bounty, but it generally seems like being a bully in the zone works.
5. 
   
6. Eventually, up the three people will receive a bounty on their head. At this point those players will receive a push quest for the FoS. It will ask you to kill 10 opposing faction players without dying.
7. 
   
8. Stay grouped up. Keep in mind you are now on the world map as a target and have no stealth advantage. Keep your bounties alive and pick off players. You only need Honorable Kills for the quest, not killing blows.
9. 
   
10. At a certain point you will need to clear the bounties, even if you complete the quest it will not push it to others in your party without "resetting" the bounty system. It appears to be random, may take a few tries before all people are able to get the bounty, and therefore the FoS.
11. 
    

Some additional notes:

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• Dying while you have the quest for the FoS will fail it. Our player who died with it said it only showed as failed once he opened his quest to look at it. Abandoning it will allow you to get it once you get a bounty again.
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• You can get bounties in a raid group. You will not not be able to get credit for the quest however, in a raid. You can group in a larger group and then break off into parties once you start getting bounties.
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• It took us about 20 minutes of bullying to start getting bounties. Not sure if it depends on amount or frequency of kills, but we were killing at least 1 person a minute.
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• 110s, especially geared ones with legendaries, can be much stronger than you think and can compete with a fresh 120 no problem due to odd scaling by Blizz. Take care in 1v1s especially, you can get shredded.
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• Once you start attracting attention, you will want to start roaming. Don't let yourself get surrounded. People will start banding together to stop your bullying (rightfully so). Try to set your own engagements and stick together, especially once you get bounties and can be seen.
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BIGGEST THING PEOPLE DON'T UNDERSTAND: You DO NOT start getting this Feat of Strength once you are an Assassin. You need to have the "Bounty Hunted" debuff (you will have a red arrow debuff) that allows players of the opposing faction to see you on the map. Simply keep killing once you are a Bounty. A quest will pop up with the title as your reward.

I got this with no preplanning. There was a huge alliance raid formed at the Tortollan emissary turn in, so I joined the raid group and killed what must have been over 100 horde.

I've gotten the bounty before while solo, and it is FAR faster to get by yourself or in a small group. In a raid it will take a while. (Failed achieve as I logged out and the quest failed).

Even though this took a while, I was 100% guaranteed to get the Feat of Strength with a massive raid and lots of horde streaming in. I simply left the raid after getting "Bounty Hunted" and made sure to stay within range of healers.

IMPORTANT POINT: Do not get greedy once you're a bounty. Be sure to only engage what will actually die and remember, only honorable kills give you credit. This means players who have just landed off a flight path, for example, will not give an honorable kill during the 20 second debuff.

The Achievement appears to be account wide, I.e. you will get The Alliance Slayer when you unlock this on your Horde Characters, However the title will only be available on the character which you unlocked the achievement on.