User blog comment:EmpyreanSmoke/Weaknesses/@comment-27036944-20151002015940

If the theory of Leviathans being able to easily kill archangels is based on the fact that Imperiex or Empreyan said Death said Leviathans can kill angels, but he didn't actually say that in the episode. His exact words were "Long before God created Angel and man, he made the first beasts -- the Leviathans." He said he was entertained by them, but God locked them away because they would eat the whole petri dish: "I personally found them entertaining, but he was concerned they'd chomp the entire petri dish, so he locked them away." Not only does he not say Leviathans can kill angels, he definitely doesn't mention archangels. The petri dish was God create life, the earth, and everything else, not simply angels.

Castiel later said in the finale that Leviathans can kill angels, but that was in direct reference to the fact that a garrison of angels was all dead. This wasn't referring to archangels, because there couldn't have been any archangels there at the time, because Raphael was dead, Gabriel was still thought to be dead, and Michael and Lucifer were still locked in Lucifer's Cage. So Castiel was only referring to lower angels, not archangels, as no archangels could have been in the garrison as far as we know.