I think only celestial Phoebe creates trouble for more players. Viewing every beasts during attack kills the excitement and a surprise factor. So how about creating beast slots. Eg lets say 2 slots. So if a player fills any 2 beasts, he can only preview only those beasts. Castles with other beasts will show nothing and give no information about the beast.
To change the beast in slots Gems can be charged.
This will bring some excitement during attack.
In given scenario after sometime, more alliances will have only phoebe because most will be avoiding to fight phoebe and fighting phoebe will be monotonous.
So to give some balance to phoebe less alliances, to equate the power of phoebe instead of nerfing it down , give other beasts the surprise factor which is entirely chosen by the player.
This will create more variety in defences and makes other beasts also useful.