Let's double-click our Enemy actor, and create a new rule. Rename it to Destroy and change the settings to Actor receives event | overlaps or collides | with actor of type | Bullet.

We want to add some health for these baddies. Under the attributes box of the Enemy actor, click the + button to create a new attribute, rename the attribute to Health, and change the value to 4, as shown in the following screenshot:

Now, drag in a Change Attribute behavior, and change the settings to Change Attribute: | self.Health | To: | self.Health-1.

What this will do is remove 1 health each time a bullet hits it. Easy enough, eh? Now, add in another rule, rename it to Die and change it to Attribute | self.Health | = | 0.

Then, drag in a Destroy behavior.