The Alliance Slayer

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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• Classes with high mobility and CC are desirable for keeping enemy players from getting away. Druid, DH, Monk, Mage are all good choices for chasers.
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I can confirm that if you disconnect while on the quest it will also fail it. Sigh...

I got the achievement with my Rogue. Only a Mage defended me.
After getting the achievement, the mage opened a portal to Zuldazar and the bounty was still on me. (I did the achievement in Kul Tiras.)

So i can confirm:
- You can use mage portals to Zandalar or to Kul Tiras.
- You can use the [item=141605].
- You can even use your Hearthstone (if your Innkeeper is in Zandalar on in Kul Tiras).
- You CAN'T use portals to old zones (Orgrimmar, Thunderbluff, Silvermoon, etc...).
(I don't know if you will keep the quest because i tested these things after i completed the quest. But you'll keep the bounty debuff for sure.)

How to survive?
- Get some players to defend you.
- Get the Artifact Fishing pole with the [spell=188042] trait.
- Stay always near of the water.
- Stay in Stealth (if it's possible).
- Put the Artifact Fishing pole to your Action Bar (so you can fast switch your weapon).
If there are too much enemies, you can easily escape by switching your weapon to the Artifact Fishing pole and swim away.
I recommend doing the achievement in Kul Tiras because there is a lot of water.