Everyone is saying that Shadow Tag Shandera will kill Blissey. Well, first of all, most people I've battled use Flash Fire over Shadow Tag, so in most cases you could just switch out. Second, Shandera has five weaknesses, Water, Ground, Rock, Dark, Ghost. Those are the most common attacking types in the game (besides fighting and fire, which it is immune to) Third, it ain't that bulky. I mean 60/90/90 is decent, but who would put EVs into those stats? I usually 2-3HKO it with a neutral attack, and 1HKO it with one of its many weaknesses.
Shandera isn't that good. Sure, 427 max sp attack is great, but Marowak has 568 attack, and it was in NU last gen. Even Medicham is more powerful, and it can work off a 205 power Hi Jump Kick. Let me ask you, how can a special attacker stop Blissey? Medicham and Marowak completely pwn Blissey (of course, Game Freak decided not to give them Shadow Tag), and all Shandera can do is calm mind. Why not just use psych up and shadow ball/ hp ground?