Basic attacks by monsters are by default neutral, so that is the reason why raydric is the most common and useful card to put in your garment, it will reduce normal attacks by 20%. However, using a card like Marse is only for reducing water property attacks (storm gust, water ball, etc.), not normal attacks.
Elemental armors work differently. They make you level 1 element which means that it reduces the damage of elemental attacks by 75%. So if you wear swordfish armor, water ball or storm gust will be reduced to 25% of its original damage. In the case you wear marse AND swordfish, the elemental armor takes priority and the marse reduces from there.
Elemental armor (swordfish) - 25% damage received from water (75% reduction) * 30% reduction from water (0.25 * 0.3 = 0.075) which means you will only take 17.5% total damage from water property attacks if you wear this combination of gear, (25% - 7.5%) so this means your total damage is reduced by 82.5%.
Edited by tehPope, 24 September 2017 - 04:32 PM.