2D top-down shooter AI

Ok, so here's the deal.

Me and a friend have this great idea for a top-down shooter but are stuck at the AI. I've been searching (and experimenting) a lot to find the secret behind the AI from Crimsondland, but are not able to crack the code.

Video of the game: /watch?v=_sd6d3CBa2g

I tried to make it so, that the enemies would attack the player directly, that didn't work because the enemies would just stack up in a big pile. Then I tried to make it so, that the enemies would walk to a random point on the map until the player got near them, but that didn't work too because it didn't have the right fell to it.

If I'm not specific enough, then pls tell me, and I will try to answer any misunderstandings. Thanks for your time.