Immortal shard golem with sniper ability and unstoppable. Doesn't die to lightning or explosion, kills massive dragon (in about 500 years) and doesn't die to fire shield.
You need to have 1 shard of integrity, 2 shard of focus (unstoppable), 3 shard of freedom (sniper) and 2 shard of wisdom (immortal). (8 cards total, so it is possible)
Tried it here to make sure:
http://elementscommunity.org/tools/xenocidius/shardgolem/ It also happens to be a 19/34 airborne creature, assuming upped shards.
So shard golem (shard of integrity) is the most powerful card in the game, since it beats all 4 conditions, at least in certain circumstances
edit: hmm actually, doesn't even need to be immortal to beat all those conditions, since it has way more than 5 hp.