You could use the system condition Pick random instance (choose a random instance, in this case you want a random instance of the family):
"Pick a random instance from the currently picked objects. In other words, if Pick random instance follows another condition, it will pick a random instance from the instances meeting the prior condition. Otherwise it picks a random instance from all the instances."
Store the Ennemies.HP value somewhere (since you picked a random instance, it ll be that instance that is concerned)
Then pick all instances of Ennemies, and set their health to the value stored.