Party of Six

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2. Kill everything before starting the fight, this resets the friendly npc's health, since they can't be healed any other way.
3. 
   
4. The special action button, if used by two people on the same target, instantly kills it and resets the cooldown on the special action button. With some coordination, you can instantly kill all the adds one by one while the third person attacks the boss.
5. 
   
6. Simply tanking the adds and getting their aggro while you dps the boss works too.
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Here's a video. Check, please?

This is likely to be a nod to the old TV series Party of Five.

Killing all the mobs before you talk to the NPC does make it a little easier, but you can still fail the achievement if your dps on the boss is not fast enough.

The safest way is to clear the area, start the event, and pull all the adds off of the NPCs as soon as they spawn, AoEing or cleaving them down as you dps the boss.

If you have a tank, it's super easy, but if not, you can still aggro everything very easily by switching to the adds when they spawn.