Elemental Lifeforms are enemies made purely of one element. They are immune to their own element. Because they are beings made out of the respective seven elements entirely, Elemental Reactions will occur when they are inflicted with a different element. Elemental Lifeforms include Slimes, Hypostases, and the Oceanid.
Mystical Beasts are similar to Elemental Lifeforms, but are not made out of pure elemental energy making them distinctly different.