How do I kill him?

Hint 1 of 4:

Well, one of the attack keys will shoot a flame out of the charged relic.

Hint 2 of 4:

Attack the dragon with it.

Hint 3 of 4:

Try to get in a good view, if you can.

Hint 4 of 4:

It'll take about 8 to 9 hits to kill him.