Cannon Confounder Achievement
Chapter 8 - Complete the level without getting directly hit by a cannon
During this stage, you'll attempt to take out the shield. As you approach, there will be three stand-alone cannons, which you must destroy to fulfill the "Take Out the Big Cannons" objective. At the end of the stage, you'll need to zig-zag in front of an emplacement of cannons as you approach the end of the stage. You cannot be hit by any of these cannons to get this achievement.
You can destroy the big cannons by tossing a boulder or similar large object at them with a Giant Skylander, or by tossing a bomb at them with a normal Skylander. Be sure to stand out of their line of fire when you do so.
You'll need to cross the emplacement of cannons three times, once going right, then left, and right again. Before moving, analyze their pattern of fire, and move when comfortable. There is cover along each path, so you can take a brief respite as you go. Just take your time and do not get hit. You'll unlock the achievement after the final battle, just before the stage ends.
Here is a video by 2-4 Gamerz, which begins with the first big cannon: