I'm trying to defeat Dustflier on Mission Mode, but I can't. He keep using his shockwave and explosive fireballs at me. Not only that, his defense is so high, I can't barely damage him. And I'm using Zero Gear! Is there any easy way to beat him?

One does not simply easily beat Dustflier.

There's no easy way. You need to find a way to dodge his attacks, and strike quickly before dodging again. It should only take you about three weeks.

it's a basic pattern. Just glide around his attacks, once the attack is done, get a few quick hits in. Try to time limit breaks if you can

that's about it

If you are at a very high level, you have Master's Circle (which you get from getting 358 crowns), and a lot of patience, you could get through it.

 

Master's Circle is amazingly useful! You cannot get inflicted with status effects, which the Dustflier loves to ruin your life with, and you only take 1 damage from each attack, regardless of its power.

All of the resistances are 100%, however, there is a minor setback with such greatness. If you miss an attack, you will take damage. And if your HP runs too low, it prevents you from healing, so watch out. If you are a skilled player, this ring is just for you.

 

Dustflier wise, just dodge his attacks and glide around. He jumps and spins alot so heal often. But to tell you the truth, there is NO EASY WAY TO BEAT THE DUSTFLIER. especially in Mission Mode.

Hey I found this 

I don't know if it will even work for you since you have to do a boss rush. But I suppose you could give this a try? 

You dust it with a duster. That's how you defeat it.

 

Really though, you need to get the Master's Circle and he willing to spend a lot of time trying to defeat it.