Don't open any chests in Lowtown, The Royal Palace, the place where you get your stuff back after escaping the Nalbina Dungeon, or the Phon Coast area with a large square of multiple chests. Do that and you're guaranteed not to ruin your chances of getting the thing.

Also, what Mullet said. I can't stand this "let's make a game for the purpose of having a guide" idea. It's bollocks. I buy guides for games that I find amazingly brilliant, not ones that wouldn't be 100% without them.