Well.. hide them with frames or sticky pistons. And DON'T just use one or two. As I said, they are SO cheap, you can build up a dozen or two very easy. Spread them along the whole mountainside and hook them up to a nice timer, like ev. 2 seconds. I would like to see you walk through that. Really.And fire a projectile in a straight line at a crawling speed, complete with a warning sound. You should consider yourself lucky if you manage to hit an AFK person with one.
Let's try this in creative. Sounds like a fun thing to build.