Start at the edges and work your way in. Meaning, don't fly through the center of the field. Destroy edge acks along one side from long range, get out of range, work your way back in upon the other edge, never letting all guns fire at you at once.
Use a cannon bird with good ballistics and lots of ammo. Rugged helps too. Typhie is good, so is C Hog.
Don't be shy about firing from maximum range. Only 1 or 2 20 mm inside of the gun pit is going to destroy the gun. Start shooting at each pit at the farthest distance you think you can make the shot.
Don't bother with rockets until you get really good with them. Cannons are much easier to learn with.
Once you get comfortable with your aim, you can actually squeeze a burst at a gun pit from long range and, while the rounds are on their way to target, you can already begin to realign yourself to take your next shot. Again, those gun pits are destroyed pretty easily, so once you get a good burst that you're sure is going to hit, immediately start lining up your shot on the next target.