Ah yes, you've met the Brimstone Cyclone. Definitely one of the harder bosses.
Like the.... er.... uh.... er..... Ok, I cant remember the name of these things, the floating "spark" dudes that are in the Craggy Highlands, kinda like a super-esper that uses Rockslide.... those guys have a field that pushes away all projectiles fired at them. The Cyclones have basically that same ability, but it actually seems a bit stronger on them.
There's a couple of things you can do to beat one of these:
1. Use melee spells. But: You have to be specific about WHICH ones you use. Some melee spells simply wont work on it, stuff like flamethrower/geyser, or either of the whips, the little mini-projectiles that make up those spells simply wont reach the boss. Instead, use things like Ice Cross, Circle of Fire, or the wonderful Insect Orb: these spells dont really use projectiles, they simply spawn effects in specific places (which cannot be pushed away by the field).
2. Use a really strong projectile speed enchant..... like, 70%, is the level I'm thinking of..... and then hit it with Ball Lightning over and over, or similar spells. Plasma Bolt or whatever it's called might work in this combo as well as it has a high speed to start with. This trick makes the projectile so fast that the field cannot lower it's speed enough to push it away....... but you still have to fire at the boss from a relatively close distance.
3. These things, combined with Bear Traps (to stop it's movement for a time).
It's a pretty common boss, actually, so get used to seeing it
