Monster/Weakness/Affinity

Hippogryph-Earth-Lightning
Gnat-Earth-Lightning
Cactuar-Earth-Lightning
Wolf-Earth-Lightning
Jellyfish-Lightning-Water
Giant Crab-Lightning-Water
Eft-Lightning-Water
Slug-Lightning-Water
Ahriman-Water-Earth
Coblyn-Water-Earth
Doe-Water-Earth
Antling-Water-Earth
Bomb-Wind-Fire
Goblin-Wind-Fire
Boar-Wind-Fire
Opo-opo-Wind-Fire
Revenant-Fire-Ice
Imp-Fire-Ice
Wight-Fire-Ice
Buzzard-Ice-Wind
Flytrap-Ice-Wind
Dodo-Ice-Wind
Puk-Ice-Wind

-=-=-=-=-=-=-=-=-=-=-=-=-
INNE PODEJŚCIE :)
-=-=-=-=-=-=-=-=-=-=-=-=-

There is a rule to what element (and its weakness) a mob is depending on the shards that it drops.
Each mob will drop 3 different shards - you can check this on YG. Now look at the elemental wheel:



Lightning>Water>Earth>Lightning
Wind>Fire>Ice>Wind

The 3 shards that it drop will always be adjacent to each other on the wheel. The element of the mob will be the middle of the 3 elements and so you can determine its weakness.

For example the puk family will drop wind,lightning and water shards regardless of what type of puk it is. Match these to the wheel and you will see that wind is between lightning and water, so puks are of wind element. Their weakness will be Ice.