Take down the Giant.
+2 for retrieval, +3 knock out Giant using slingshot.
You must first make it to the Giant's Cloudland. Although difficult to gather from the image, the slingshot is in the hole in the tree (circled in image to left).
In order to make use of the slingshot, you must have some pebbles.
The slingshot can be used to knock out the Giant and get the Magic Chest.