How to get the "Nyuck, Nyuck, Nyuck" achievement?
Solution 1:
Get level 60, skill some AOE skill, like Blizzard for Wizard, Spiked Trap for Demon Hunter etc. Open a game on Normal difficuly. Start the quest. Rally them up. Just let them close in and hit you, then run away. They will all follow you and cluster. Then use your skill and kill them all at once.
10k+ DPS will kill them with one blow.
Solution 2:
Demon Hunter Rapid Fire counts as "one blow" (for example can also get mighty blow callouts). Level 36 DH (1k DPS) playing Normal got all 3 drowned temple champions in less than a second, and the achievement.