Increasing a creature's element will both increase its offensive capabilities when utilizing said element and defensive ability when attacked by that element.
Keep in mind that the game makes no distinction between elemental healing and elemental damage.
For example: If an Efreet has 30% Fire and an Marid has -30% Fire the Efreet will receive a 60% boost to its damage against Marid IF the following is true:
If the Efreet uses the skill "Dragon Fire", his Fire affinity, which is 30%, will be added to the skill's Fire affinity, which is 10%, thus giving the skill a 40% Fire boost.
|Complete Elements List|
|Increasing Darkness will decrease Light and vice versa.|
|Increasing Fire will decrease Water and vice versa.|
|Increasing Earth will decrease Wind and vice versa.|
The vast majority of equipment will increase one specific element's power and at the same time decrease the opposing element's power, functioning much like a double-edged sword.
You can increase every element a creature has by increasing MGC instead, since it raises all elemental affinities equally, but slowly. For example, both Fire and Water will be increased by MGC.