Gunz?? bringing down a dragon???
*ahem* basically the only thing gunpowder would be better than would be *mundane* things. Magical Beasts, Magical Shielding, Enchanted Armor ... all of these things would be largely unaffected. A stone shield, for instance, could protect against many bullets. Also, water magic would temporarily disable all gunpowder based units for a certain amount of turns (say, if it was a *FLood*, *Hurricane* or *Rainstorm* attack, then all of the gunpowder would be disabled for at least a good 3 combat turns if not more. Once more, fire attacks could inadvertently (or purposely) trigger an explosion of gunpowder cache's.
Guns would be more expensive than Bows (probably) but would require little to no training, so overall troops Time would mean Gun units could be produced faster. However, even a little bit of Fire or Water magic can turn these troops some-what useless (or dead). Therefore, if you have enough gunpowder, you can simply Crank out hordes of these gundpowder units, and hope enough mooks make it to the enemy before they are exploded or soaked in water. I would start by saturating the battlefield with Oxygen, equipping Fire Shields, and spark the gunpowder/oxygen mixture with some minor Fire Spells. Then I would Duck and Cover when half of the map is covered in Flames, and bye bye goes 2000 musketmen. Cannons would be trickier I admit. More resistant to Water damage and Fire damage, longer range, and far more powerful Kick. The weakness, of course, is its slow movement. AND massive amounts of metal involved. A couple Well Placed Electrical Attacks would destroy them quite well.
Essentially you would need a good bit of shielding to protect your volatile new weaponry, so it would still require magic. In any event, perhaps anyone going this route should be met with Magical Penalties of some sort.
In any case, the more I think about it, the more I am against using blackpowder in this game 
