Give all his worshippers the ability to cheaply and easily create kevlar vests? (or any other armor, introducing armor as DR for his followers)
Give everyone the ability to cast Create Water at range specifically targeting gunpowder.
Give everyone Invisibility 1/day.
More important than anything, what are the levels and classes of the gunners and the orcs? Levels and classes are really important in this game. Anything else depends on this answer. If Clangeddin gives his followers guns and Bane just ordains all 3000 as clerics or whatever, it won't take many levels before the gunners are screwed.
What sort of information do the various parties have on the other? How much preparation time does each side have? Where are they facing off? Again, we need to know all this to help.
Have the orcs use their brains. If they mass up and charge they will die. Maybe they invent trench warfare. Maybe they choose terrain where the cannons can't be used effectively.
And depending on the age and color of the dragon, 400 guns are easily handled. Give me a great wyrm red and you'll have a lot of dead gunners.
This might be the easiest solution.
