...They All Fall Down

Very Easy for Battle for Azeroth Level 120 and beyond...

1a. Heroic Difficulty.

1b. If you are taking severe damage for some reason, equip a Legendary that heals you, or just heal yourself if needed.

2. Aggro Boss, do not fight and move to the side, away from the center.

3. Go do something else for 8 minutes.

4. Wait for boss to teleport away. You should have 12+ mobs on you.

5. AoE Kill all 12 mobs instantly.

6. Kill Boss.

7. ?

8. Profit. Achievement Completed.

Step zero: Be 101+
Step one: Equip Prydaz (The legendary neck every class and spec can use), Roots of Shaladrasil (Mail legendary pants), or be a druid in resto spec for Ysera's Gift or any Azerite armor with the regenerating shields or on-damage heal effect.
Step two: Aggro boss
Step three: Go do something else for 10+ minutes
Step four: Wait for boss to teleport off to summon more adds
Step five: AoE the adds.
Step six: Kill boss.

Edit from THE FUTURE: By 116, you've outleveled the legion legendary options, but by 120, you now have azerite items, many of which provide survival traits. Also Ysera's Gift is just inherent for resto druids rather than talented. Edited previous steps accordingly.

All adds counts as protectors, the older posts stating otherwise are seemingly outdated. Of course it's easier to just keep defenders around, but all adds count.