Google "Construct 2 dictionary", there are a few good tutorials.
Put Enemy object and Skills dictionary to a container and you'll have a unique skill set for each enemy.
Your entries in the skills dictionary could look like this:
Key: Fireball Value: 3
Key: Poison Value: 0
Key: Heal Value: 5
Writing an AI for enemies should be an interesting task. You can start with something similar to this:
If enemy.health<50 & Skills.Get("Heal")>0 // then use heal spell
else If Skills.Get("Fireball")>0 & Player.fireResistance<50 // use fireball spell
else If Skills.Get("Poison")>0 & Player.poisonResistance<50 // use poison spell
else // use normal attack