User blog comment:HELO/Le-WHY-athan/@comment-4950455-20120908055305/@comment-5287758-20120908062025

I agree with Zxjkl. The Leviathans aren't invincible. They were caught by surprise back when their purge began. There could be a number of reasons why they couldn't defeat the Reapers. The power to take down a Reaper capital ship(how they pulled it off is still a mystery, must have something to do with dark energy and super-strong biotic fields), maybe they didn't have it back then.