I would recommend to keep fighting FG, because with that many upgraded cards you should be able to win... probably you just had a really unlucky streak... but I'm pretty sure with so many upped cards you should on average earn more than you spend.
If you lack money to fight FG, fight something easier a few times then fight FG once again, you will win eventually.
Did you lose 700 electrum due to not winning a single time, or did you win at least once, but without winning a card?
Did you face mostly hard FG?
Because it could either be simply bad luck, or maybe you are doing something wrong with that rainbow.