well, is something like this my idea:

earth - use strong and power atacks, like a punch, a kick or a tail strike. One of Darko's power is tha he can raises a stone wall from the floor to use as a temporal shield.

water - Eilyn can use the element of water in her favor, like making a big wave to strike a ship or a boat, open a way in a small lake or a swamp, use the water itself to strike the enemy, use the water to cure some wounds, etc.

fire - Ray can exale fire from his mouth, he can control that fire to use in his favor too, like making fireball from that fire, he can cover his swords in fire for a more stronger attack.

air - the dragon for this element is still in development, the dragon will control and use air in his favor too, like moving things with air, make strong winds at the enemy to push them out of range, make airball that can explode like a bomb but with air, this dragon will be a spellcaster of wind.

All the four dragons have a weapon and a combat style which can involve an element in they fighting style.