Challenge battle 5 is generally regarded as the hardest battle on standard difficulty. It can mostly be done with normal gold cards (without promotions or good gear), but it will take a lot of practice and luck.
Challenge: Power Surge!
Challenge: Countdown! (2:45)
This boss is of unusually low stats for a standard challenge boss. While this match often requires a skip due to the spike in difficulty, it is likely not needed here.