You’ll notice that both Light and Dark side battles contain a Normal and Hard mode. 

Tiers 1-3 of Light Side battles are available immediately, but you will need to complete all of the Tier 1 Normal Mode battles to unlock Tier 1 Hard Mode battles. 

For Tier 2 Hard Mode battles, you will have to complete Tier 2 Normal Mode battles, and so on for the rest of the Campaign.