I carry around a shadow and an elemental weapon (and pierce as soon as I can get one) and know all the enemy weaknesses. Some enemies are not taking weakness damage. Ones I have personally seen are:

Gremlin Menders
Gun Puppies
Mecha Knights
Spookats

I have damage numbers turned on, and know about how many hits they should require to kill for the first half of Tier 1 (1 hit from a Faust or Divine Avenger, I was working my way down so I wouldn't have to pay to start from the Manor). They were taking the correct damage yesterday, the 26th, but today they're just taking normal damage. In Tier 1 this is obviously not a huge issue for most people... but in the lower Tiers it could be quite a set back as everything just hits harder and you want to kill the enemies fast so you survive.