I checked on reddit (I was that desperate!) and it boils down, to me, to those missions being probably tuned for Chinese whales, making even our own whales suffer against those challenge. You need a full 6k cast to stand a small chance in those last two it seems, since tactics or even cast don’t matter, only raw numbers.
Frankly, I am probably done with the game if that’s the new content we can expect from now on.