I think this is a good reworking of the achievement. I've heard a lot of horror stories about flawless Undying/Immortal runs ruined by a disconnect or a lag spike. With the Ulduar achievements, at least bad luck will only bork that one boss. And congratulations to We Know Girls!
Update: As pointed out below, VoS on the Sargeras realm and Bastion of Uldaman got the achievement May 9th, last Saturday, which means We Know Girls was not the world-first (due to the Armory's erratic updates giving GuildOx the We Know Girls information first). Grats to VoS and Bastion. Anyone know of any earlier than May 9?
Update #2: GuildOx now properly reflects the order that these guilds achieved Champion of Ulduar.