So, for the last time. Bonuses of same type (e.g. against demihuman) add up, so three hydras would do +60% damage. If you have two or more bonuses of different type, they multiply, so two hydras and two skel workers will result in +40% dmage * +30% damage, so calculation goes this way, 1.4*1,3=1.82, that means you get +82% damage against demihuman medium monster (players).
For elements, level of elemental armor determines the damage multiplier from the element of weapon/spell. It works like another damage mod like those above. It does not add special bonuses to +% damge on element cards. So for example...you have weapon with +40% damage on water. If you attack with neutral element, the damage will be 140% no matter what level of element is the monster. If you get endow and your attack element is wind, the damage on water 1 monster will be 1.4*1.75=2.45>+145%, while on water 3 monster it will be 1.4*2=2.8>+180%.