This is one fault of the game... there's no way for you to know about platforms above or below you unless they're basically right there. But if you look around, you'll see that to your right between the two high platforms there's like a 19 foot drop onto another platform with another bucket and a monster. Just take the bucket and either sacrifice a single life, or drink many potions beforehand and just grab it and drop off.
After that, look up. Eventually you'll find a bunch of single tile wide platforms going straight up with items along the way, but the exact coordinate eludes me at the moment. I was just lucky, because there was a gem on the first one when I played through the level so I knew it was there.