How are we power leveling these days? I've searched around and things have changed so much since I went on break.

Is there an ideal zone where I can stack my 2x and 5x and get massive xp? Is there a regenerating open zone where we can just stay for the length of an elixir and hack and slash and chat like we know what we're doing?

I went into this The Overlord Returns map 1 called "A Realm in Ruins" and there were these aliens but they were gray and my EASY kills got me no xp. I'm like... what?! There was one easy-to-kite mini-boss and I got a few xp for him, but not for any mobs. So I'm open for any advice.