Once Bitten, Twice Shy (25 player)

Source
is the final boss of the Crimson Hall wing of Icecrown Citadel.

Strategy
Not a particularly daunting achievement, but it can be something of a pain to get. DPS will have an easier time defeating her while becoming a vampire and healers and tanks will have an easier time defeating her without becoming one. It requires either to have duel specs (one tank/healer and one DPS) or some communication with your raid leader requesting that you be bit or not be bit. Other than that, there is no specific strategy or quirk to the tactics of the fight to get it.