...sure. Use trigger zones to spawn monsters as needed.
By right-clicking on the trigger zone, you can finely position it, much like an entity, and place it exactly where you want the player to be, when a monster is spawned.
You can have a monster appear right in the player's face, or creep up immediately behind, for an ambush.
Ambush from behind is most effective, because the player has already been lulled into a sense of false security...he/she has already seen the area directly behind them, and hasn't seen anything.
Fine tune the trigger zone technique. You'll come up with a way to scare the stuffing out of even the most confident player.
Widow 13 - Coming Spring 2008