This is why I mentioned that the opponent power should depend on your actual level/strength, rather than just a blanket difficulty that caters to a wide range.
That would allow all levels of the Battle to be appropriately difficult for everyone, rather than level 125 players potentially struggling to complete and level 150 players breezing through with 1-2 heroes.
It could for example make a calculation based on your maximum Team Power, which we know exists as a stat, and apply an increasing multiplier of that calculation to the boss battle opponents' power for each level.
Or even, assuming 30 heroes to complete a level, it could look at the total power of your top 30 (the code must be there since Team Power gets the top 5) and use that for a calculation.
If I were on the team these would be the sort of things I'd be looking into, anyway