Just keep hitting it with whatever hard-hitting weapon you have left, it will drop down. Careful with satchel and nades, they are excellent for taking down baby headcrabs all at once, but it is easy to blow yourself up if you arent paying attention.
Save your Crossbow and Magnum .357 rounds for the next few levels. They are the most bizzare (and IMO, most awesome) levels to the game, but there are a LOT of floating Xen Masters. You need an accurate weapon with a lot of dropping power to rely on taking them down fast, hence the bow+magnum.
On another note, I used to be god with the magnum, especially in multiplayer mods using the original game, such as SvenCoop, or Deathmatch. However Natural Selection>All. Like it matters now, though
