I Choose You, Stormcaller Brundir (25 player)

Obviously a reference to "Pokemon". Whenever a trainer would send out a pokemon, they would say "I choose you <name>!"

I found it pretty funny. :)

Killing a boss of the Iron Council Supercharges the remaining bosses, healing them to full, increasing their damage and giving them new abilities. To get this achievement, Stormcaller Brundir must be killed last. Brundir can normally be stunned and taunted. If both other bosses are killed, he can no longer be taunted. Killing Brundir last is the easiest approach and does not grant aditional loot.

Brundir starts off with the interruptable Chain Lightning and uninteruptable Overload abilities.
When 1 other boss is killed, he can channel Lightning Whirl, shooting bolts at random targets every second.
When both other bosses are killed, he will cast Lightning Tendris, lifting Brundir in the air as he follows a player, doing AoE damage. He resets aggro once he lands, so DPS must be very careful since he also becomes taunt-immune when both other bosses are down.

All you need for this Achievements as a level 100 just kill first [npc=32867] and [npc=32927], and after kill [npc=32857] (smallest one)