Make sure you're using the Zero Gear with 4 Sight Units and the Critical Sun ring equipped. Otherwise you won't be doing any damage to it.
This means you must beat the game first before tackling the Dustflier.
As for tips:
- Glide, glide and more glide. No matter how close you are to the ground, his shockwave can't hit you if you're gliding. Stay close to him so you can attack him when he's down, but far away enough so you don't get hit by anything.
- Wait for him to Shockwave. Glide to avoid this. Afterwards, he'll either:
1. Do a somersault. This does a shitload of damage if you get hit, so never stay close to him after a shockwave. After he does this, go and whack him once or twice (you won't have time to get in three hits).
2. Charge at you. Use Air Dash to avoid this. Then whack him a few times.
3. Shoot fireballs. Hide behind either side of the stairs, the fireballs can't hit you there.
- Aim for his head, it will do the most damage.
- Don't bother stocking up on Attack magic (unless you need them for the other bosses). Dustflier is pretty much immune. Brings lots of Cures instead.
- You can kill the Shadows for HP.