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Wish I could upload Capx, but here is an example of what you could do. Create an invisible spawn object that moves around randomly and spawns a zombie every five seconds or so. Also have an invisible player field pinned to the player, if the spawn overlaps that field then it moves else where to spawn the zombie. I am fairly new to construct myself but I did this when playing around and it worked for me!
Hope it helps otherwise I am sure someone else can get you something even better. <img src="smileys/smiley4.gif" border="0" align="middle" />