In the first part, the key to defeating Mordroc is
to use the "green arrow" that is located in your inventory
(press Start). Select the green arrow and lock onto Mordroc.
Fire the bow with the green arrow five more times. You have to
replenish your green arrow one at a time by killing the mini-black
dragons that Mordroc fires at you. Keep collecting the green arrows
and make sure to lock onto Mordroc.
In the second part, once Mordroc goes down upon his knees, he
changes into a Black Singe. All of your elements are taken by
Mordroc and you are going to have to fight the old-fashioned way.
He flies around shooting lightning bolts from his mouth. Run away
every time you see the electricity forming around his mouth. Get
into your guard stance when you do not see the electric bolts
forming around his mouth. In another words, Mordroc the Dragon
shoots out a ball of electricity (instead of the stream of electricity),
and you have to catch it by being in your guard stance. Once your
sword catches the bolt, release the bolt and use it against Mordroc.
Once Mordroc the Dragon falls to the ground, run up to him and
strike his stomach with the sword. The process then repeats. Remember
to keep a close eye on his mouth -- electricity forming around
the mouth is bad and no electricity around the mouth is good.
Do this five times and enjoy the ending sequence.