The safest way to complete this achievement, is to either bring two healers, or one healer and a backup healer, such as a hybrid casting class (Moonkin, Shadow Priest, Elemental Shaman), in case the main healer gets Frost Tombed.
Frost Tomb deals 2000 damage per second over 18 seconds. This means that a Frost Tombed player will have to be healed for around 40,000 damage over 18 seconds, while the healer is still keeping the tank standing. Because of this, having two players able to heal is almost an absolute necessity. If you do not bring two healers, then luck will play the most part of this fight. If a person dies to Frost Tomb, you will not get the achievement.