Log Lifter Achievement in Skylanders Giants
Chapter 3 - Complete the Log Lift Feat of Strength
How to unlock Log Lifter
The log lift feat of strength is found toward the end of the stage after Brock destroys the bridge you're trying to cross. After the cut-scene, walk toward the screen and left, where you'll drop down to another area. Use a Giant Skylander to approach the stack of logs, and press until done.
Here's a video by XboxAchievables: