I vote for a less grindy way to do this. Not some "defeat thousands enemys to win lotsa rares and lotsa money and waste Half-Life"
I mean, that's not overly much, this double mark. Maybe winning against EVERY SINGLE FG 5 times without any losses in between, and a progress shown. The losses inbetween only cut your progress for one FG, so it is like this
Win against Miracle, win as before, win against Morte, loss against Scorpio, win against Miracle, win against Morte and loss against Morte gives you a total of 3/5 Miracle progress, 0/5 Morte progress and 0/5 Scorpio progress.
When you reach full progress on all 11 you get double mark.