First off, I think I love this image: Yes, I definitely love it. His eyes are a UNIVERSE of tears.

Second, I generally do content involving low level monsters on Beastermaster rather than Blue Mage, so I'm curious how drastically Blue Magic damage goes up as monsters go down in defense and level.

If the increase is actually as drastic as I'm remembering from the few times I've decided to cast pointy things on old content, I'd think that at least Benthic Typhoon and Delta Thrust would be worth casting due to the combined damage and enfeebling effects.

A lot of stuff does die fast, possibly before Defense Down or Plague would matter, but a lot of stuff in old content also lives a long time just by virtue of having too much damn HP.