It’s listed in the beta-beat thread:
The biggest use is in removing the NEED for the mana troops.
For example, if you have a dual family bonus, you can mana break Victor & Vanda with a Level 1 mana troop.
In the 4* arena, there IS no mana troop… So you can mana break Valaria & Frank if you get the 3x hero bonus going… For example, a 4* Bloody Battle or Rush Attack tournament team might look like:
Valaria, Valaria, Valaria, Frank, Jack (+15 or something)
Doing this would mana break all three Valaria to charge in 6 tiles (plus their DoT stacks/ resets each other to get it up to the max DoT quickly… plus decent sniping damage). It also mana breaks both jack & Frank. Frank will enhance the damage of the 3x Valaria too with his defence down & Jack’s Pumpkin bomb does ~20-25% of an average 4* heroes HP in one go… which will increase with the emblems assigned…
So there IS a use for the family bonus… Particularly when you consider that the Halloween Heroes are predominately Offence Based which is where the Mana Breakpoints are far more linear & important.
You should easily be able to complete the Halloween Event. I’ve done it previously only using 4* heroes. I believe that @madmarv has a video on his youtube channel about beating the Seasonal Events with only 4* heroes from S1.
Carpet Bombing defs helps 