A Whale Shark is practically impossible to tank because they can hit over 500,000 damage (850,000 on clothies). The only way to kill a Whale Shark and earn the achievement is to kite it, which can take quite some time. Tanks can, however, hold and continue to hold aggro by using whatever they have to hold aggro on a Whale Shark, while staying in distance to it (8-15 meters away at least). Going over 20 can result in resetting the mob or most likely going after another member of your raid/group.