Creating items that "auto-shoot"

Okay, i've played a map called battleships a few days ago and the game was based on a sea where ships fight with weapons. But it's not like weapons that you can use by right clicking. You get a item and when an enemy is in the fire range of the weapon, it shoots. So you can't control where you shoot nor who you hit. So my question is how you make an item and when you buy it, it acts like a weapon and shoots.

Afaik battleships is game totally dependant of the skill "phoenix fire".

Create a new skill based on the "Phoenix Fire" ability as Vexorian has suggested

Cooldown of the ability = attack speed
Initial Damage = Damage
Damage over time: Set this to 0
Duration: Set this to 0.01

(those last 2 are important)

Then create the item, and add this ability to the item and presto, auto firing weapons