Water Horse Kelpie

As will a good sneeze

Kelpie only inflicts damage on Fire ATT monsters. The button to use should be grayed out for all other ATT attacks, won't work in the wood dungeon you were running. Kelpie is useful for Ruby Dragon Dungeons if you don't have Ice Samurai Ogre, takes out Super Ruby Dragon in one shot. Otherwise only use I've found is feeding him to my UVO'd Ogres for skill up.

The answer to your actual question here, how to kill the kelpie, is either :(a) a guard break monster like Shiva or four-star Baron from the Knight collection, Earth Baron is ideal; or (b) a devastating attack monster like an evoed Ogre, Goemon, Seraph Lucifer with his Morning star. With 2nd group, have auto heal or a heart maker as these attacks take HP to 1. In NO RCV dungeons, you need a plan B for that health loss or make it the last move on Boss. I'm still rather new only Rank 170's, but hope this info helps someone. BTW, if Luci is fully awoken, you have some AH.

Literally as I posted that it died

I upvoted you not because of your hateful comment but because of your name
Same as mine similarly