Elfbane

Update: according to comments below, this does not work anymore. Try at your own risk!

Previously:
There is a very easy way to defeat Elfbane, regardless of ilevel or class. You just have to have the introductory quest chain to Suramar unlocked.
Inside Shal'Aran, there is [npc=97140], who once in a while craves for some ancient mana. Complete the semi-daily Feed Thalyssra and you'll get a buff: [spell=224772]. That'll prevent you from dying (once), heal you after that, and do damage.

Now go out and find Elfbane: attack him, let him club you to death, and wait for the buff to take effect. At the same time that I was being healed up by the buff (while stunned; a few seconds), Elfbane took two huge hits from the (shield provided by) buff and died right next to me. Problem solved.

This is the hardes rare of all from Legion "Adventurer of ..." achievements.
He is elite, has 15 + mln hp and deals 200-400k dmg per hit.
I would recommend you to find party or you can try to take tank bodyguard like i did.

Elfbane rare is located in Suramar 22.2, 51.8
/way Suramar 22.2, 51.8 Elfbane

Elfbane kill and location video: https://www.youtube.com/watch?v=8mHYvUWvFxE

[quest=41319] is part of the Adventurer of Suramar

General info:
Rare NPC in legion have almost instant respawn time (1-2 min max)
On first kill drops [currency=1220] and sometimes [item=138839]
[item=138839] can be buffed with [item=139390]

So, Elfbane hits very hard, between 200-400k and has a stomp ability with a short cast time that does around 800k. Pretty much impossible to solo, or even to kill with a tank. However as a rogue, the &*!@#$ class in wow he was easily killed with the follower [spell=225088] that gives you the special ability [spell=222357].