How do I make an enemy keep on spawning itself?

Create an enemy that is "invisible" (no drawing).

Next, set up that enemy so that he does an attack that does no damage, won't show the damage, and can only target it's allies. (access this from the bitsets.)

IMPORTANT: Set his speed to the rate you want to spawn. Not to mention that he is untargetable. (bitsets again)

And then, choose the spawning enemy and set the spawning to "SPAWN WHEN HIT".

Then test.

NOTE: This might be slightly limited, as the heros will effect the spawning when attacking.