User blog comment:Egyptian Hybrid/Leviathen vs Archangel/@comment-26982641-20170405074647/@comment-31647781-20170405105227

It is kind of confirmed (ish) in Death's speech in Brother's Keeper - "Before there was light, before there was God and the Archangels, there wasn't nothing.  There was the Darkness...", which kind of suggests that there wasn't much before the archangels. Plus, the archangels were created before the universe, so before Purgatory. If the Leviathans were older than the Archangels, they would just be existing in place where there were no physical planes and running round unchecked.

It all depends on the way you read it though, but I think that an Archangel should easily be able to smite a Leviathan, or at least blast a lot of them with white light. If they can't do that, then they could always create a pool of Borax or something.