In monster legends, every monster belong to a certain type called element. For example Firesaur belongs to fire, treezard belongs to nature etc. Each element is strong against one element and weak against another elements.
From this table you can see that Fire beats nature, nature beats magic, magic beats metal, metal beats light, light beats dark, dark beats earth, earth beats thunder, thunder beats water and water beats fire. So if you are fighting against treezard (nature), you should use firesaur(fire).
In combination of two elements (hybrid monster) if certain element attach is stronger on any of elements of hybrid, then the attach will be strong on hybrid monster if hybrid monster does not have same element which attach is used on him. For example, Firesaur(fire) attach will be stronger on treezard(nature). But it will not be stronger on pandakens(fire+nature) because it has same element. fire. On the other hand Rockilla(earth) attack will be stronger on thunderix(thunder+fire) because it does not have earth element which is resistant to rockilla(earth). If you have any questions please tell me in comments below.