So the basic magic is that Imagers can cause things to appear out of nothing just by imagining it. They can also make things disappear. The limitations are the size of the things they "image," what the items are made out of, how far away they're trying to do it from, and how long they keep doing it. Obviously the closer and smaller the object the easier it is. Organics are pretty easy, metals are much harder.
There are also some "secondary" abilities the nature of which isn't really clearly explained. If they're strong enough and sufficiently skilled they can harden the air into invisible shield for protection, they can make "concealment shields" which prevent people from seeing them, and they can project emotions (which is generally used for intimidation or for cajoling people.)
Past here their are spoilers for the first six or so books of the series. (Which means that no one else will be reading this post, since i don't think anyone else reads the books.)
The main character is a very powerful Imager. (Shocking for a L. E. Modesitt Jr book, i know.) He's kept it secret for most of his life (since Imagers are generally viewed with suspicion) but there's a war going on and he's been put in charge of battalion (duh) that includes a contingent of other Imagers and he's gone public with his own abilities.
They've been using Imaging in a number of useful ways in combat, but even though he's been shown using subtle means to *ahem* do away with problem people in the past (creating water in their lungs, obstructions in their hearts, or poison in their brains or blood) he seems very blunt in how he uses his powers now. Shields to block projectiles or for leading a charge and throwing metal darts at people (things can be imaged moving at speed.)
Now however they're running into musketeers and cannon. It's hard to hold shields against too many musketeers and (presumably) impossible to do so against cannon. So they're having to get more subtle in their approaches. Trying to figure out ways to assault the forces without exposing themselves to too much fire first.
However a number of times he's put up concealment shields and gone to scout the enemy positions at fairly close range. And at one earlier point when the enemy was trying to sabotage something with loads of gunpowder he and the other Imagers created red hot bits of metal inside the containers, causing them to explode.
The _obvious_ solution is to put up concealment shields, sneak around to the back of the enemy lines and blow up their powder stores, destroying the cannon and presumably putting the entire force into disarray. This does put the Imager in question at some risk, depending on the range of their power vs the size of the powder stores they're blowing up.
An even better solution would be to Image plugs of some kind into the cannon. Given the strength he's shown before he could probably spend an hour or so and plug not only the cannon but also all the muskets as well. Then go back to his troops, lead the assault, and laugh when they actually try to fire at him and have their weapons explode in their faces instead.
I've similarly wondered why when they've been facing polearms he doesn't just image away a slice of the pole near the base, causing the weapon to fall apart, instead of charging them with shields on. He's used the same tactic against individual opponents before, but in that case i can at least try to assume that he doesn't have time to do that to enough pikes to be effective during the time they're charging forward, though that excuse hasn't explicitly been given.
However for dealing with cannon and firearms he could do it quietly while under stealth and no one would realize until they tried to use them, so time shouldn't be a factor.