Hey szzg007, you have probably already solved your problem or given up by now . But just in case you still need help, I'm attaching photos of a missile behavior that will acquire a target based on closest enemy and then track and strike the target. My missile spawning behavior is a scene behavior. It will first get a count of enemies in the scene and then spawn as many missiles as there are enemies.
I did test up to 10 moving enemies and it did work reasonably well. A spawn rate of every .55 seconds and a missile speed of 50-55 produced the best results for me, but you are to able change these to suit yourself. It's not perfect, about 40%to 50% of the time with 10 targets it will have one miss, and maybe10% of the time it will have 2 misses. Misses are not a real problem since you just have to mouse click again to spawn the additional missiles to take out the missed enemies. Let me know if you have questions.