It's like oosyrag and briggybros said.
Put all your enemies in a Family called 'Enemy' and put all of your projectiles in another called 'Projectile' for example.
Give both families a 'family instance variable' (right click on the family name) with the names you want like health and damage. Now click in the layout-view in the left Objects-Windows of all of your objects one by one. If you click an object you will see on the right instance variable in the 'Objects properties'. Now you can give every object a individual value for health or damage.
Projectile| On collision with Enemy -> Enemy| subtract Projectile.damage health
The rest does construct do for you.