To get "Flawless Victory" you have to win a game without dying and not giving up objectives like Herald, Towers and Dragons.

WE DID THAT. Huge hussle, atleast for us.
It took us hours of grinding and theory-crafting only to find out the system doesn't work.

Post-Game-Lobby: https://imgur.com/aXWdBH3
Challenge explained: https://imgur.com/VL2kJ07

If a Dev sees that thread, pls give us the reward. I legit chose grinding that challenge over some happy time with my girlfriend. :(