Heading down the road in the Highway 17 chapter in Half-Life 2, you'll eventually see an overturned semi with other cars, and nearby there will be a small hill with a tree, a dead guy, and a supply crate on it. Run to the top, and there will be a crossbow. Aim at the soldier on the billboard with its scope just a tad bit high, but as long as you hit him, it should pin him. Also, in the words of AbmaOnline, if you manage to hit him and not pin him, you should get the achievement anyway.
Note that this crossbow can be missed, and that this is the only soldier standing on a billboard in the game.