I am a newb to FPSC, so may not be right, but here's my try nevertheless.
If you just want to finish the level, you can set the enemy as an objective. That way, when you kill him, you win.
If you just want to trigger some event, here is what I would do:
Make sure the enemy is in a narrow alley (or in front of a door or something). That way, to move ahead, I will HAVE TO kill him.
Then just set the trigger point a little ahead (or behind the door).
Hope it helps.
-- n00b at playing games...and now at making em too :p --