This trick only works in any screen that has an Ancient Tunnel in it (the best one is the South Coast Ancient Tunnel entrance), and the Sea Giant is North of the Ancient Tunnel. Your character must have a speed of at least 10 and have a good bow (the Elven Bow of Destruction works best).
First clear out any minotaurs near the tunnel entrance. Then find yourself a giant and lure it so you run around the tunnel in a circle. If you get too far ahead of the giant while it is next to the tunnel mouth, the giant will attempt to get back to the spot on the map where it was before it saw you. This means it gets stuck and it also means you can get on top of the tunnel and shoot it until it dies (it does take a while).
Happy Hunting!
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