jyun_yi_1012 This will be moved back to your original thread, anyway, as I said there, this has been reported, and we're awaiting the devs' response. Given it's a weekend and the game is operational, I wouldn't expect much in the next few days.
My last 3 games were a similar experience to yours, so yes, it is very annoying, especially that all of the past crazy hard challenges I typically breezed through, this one, for some reason, is out of my reach despite being in the top 250 H2H board in the past and having max non-badged OVR (122) full of the so-called prime icons/heroes trained/ranked up to max. I only managed to beat it once, so I have 1 star only.
What's your formation/tactics on scoring? It might be useful for others while we wait for the difficulty reduction, scoring 5 back isn't easy against godlike AI.