Board Thread:Questions and Answers/@comment-112.198.29.122-20141203202002/@comment-5007614-20141203203043

Adding an element to a unit's attack does just that, "adds" the element to the unit's attack. It does not affect the unit's type for defensive purposes. It does not replace the unit's base element, but multiple added elements do stack.

Hopefully an example will explain things more clearly:

A Fire-element unit will inflict extra damage to Earth units, because Earth is weak to fire. The same Fire unit will only do half the damage to Water-based units, because Water is resistant to Fire.

Adding the Thunder element to the Fire-based units attacks will:
 * deal extra damage to Earth (because the unit is still doing fire-based attacks)
 * deal extra damage to Water (because the unit is also doing thunder-based attacks)

Water-based monsters will still do extra damage to this Thunder-infused, Fire-based unit, because the attack buff does not work on defense, and it does not change the unit's type.