What should I use for this?

I'm trying to create an NPC that shoots at other NPC's but I haven't found a way to actually find the other NPC's. I thought of using a Module Script to find the NPC's that were out and see if their position was close enough to the starting one, but it seems I can only use it once. I'm new to LUA and I'm unsure what I should use for this? I don't want the full out script, just what action/event I can use to call to find the nearest NPC. In advance, thanks for any help.