How can I approximate wasabi taste?
A good portion of the "wasabi" that you buy is a mixture of western horseradish, mustard, green food coloring, and occasionally, a token amount of actual wasabi.

So if that's the experience you're trying to replicate, use western freshly grated (or prepared) horseradish and a small amount of powdered mustard.

Having eaten fresh wasabi, I can say that the sensation is more pronounced than the taste. If you're trying to mimic freshly grated wasabi, you're going to have a harder task ahead of you, but the primary objective will be to find ingredients that have a clean aroma but have a marked pungency. Freshly grated western horseradish comes pretty close, though there is a difference.