Group composition of two tanks, two healers, and a DPSer makes this achievement quite easy to accomplish, since Gal'darah cannot impale his primary aggro target. Simply hold DPS or have very light DPS and make sure that once one of the tanks has been impaled, the other tank taunts Gal'darah off and tanks him so the first tank can also be impaled.
Once everybody has been impaled, he can be killed normally. Do not worry if a non-tank gets killed by an impale, as they still count for the achievement. A healer being killed can make it slightly more difficult, but no more than usual.
EDIT: June 22, '09 - A death mid-fight, with a battle res, does now discredit you from getting this achievement. Even if the player that died gets impaled again. Not sure if entire party needs to get impaled a second time around.
July 18, '10 - A death mid-fight does NOT discredit the party, nor the player who died, from getting this achievement.