Golden Egg #31 is hidden on level 26-15 (Short Fuse).
How To Obtain
To obtain it, aim Chuck down as far left as possible in the mini passage. Then aim Shockwave Bomb so it forces left, so that Chuck gives you a golden egg. Or you can just simply fling a Hal down the passage.
How To Complete
To complete the upside-down level, aim Chuck so it knocks as much potions down as possible. These potions will effect some of the pigs. Aim Hal so it defeat the two left pigs. Then use the other birds if you need to.