This part of the wiki is *******: enemies are in fact to weak or resistant to certain elements. Check the in game "amrita memories" dedicated to the yokais : if you press r2 (i think) it shows you some stats of each enemies. It clearly states which elements the yokai is vulnerable or resistant too.
It is also important to note that the resistances do not necessarily follow the order stated in the wiki. For example, a enemy that is weak to fire doesn't necessarily have to be resistant to water or earth. Some enemies are weak and/or resistant to more than one element.
I have personally checked some of those. for example, the lesser umi bozous (the water thingies) can be one shotted by the onmyo magic fire projectile. Any other element does miuch less damage, except water, which does 0 or a negligible amount.