Almost. You fight Ermac, actually. Which round you lose is unimportant, but a mercy must be done in the 3rd. Hit him one more time (to trigger the second 'Finish Him!') and let him fall down. The current scores in MK3 for MARP use the wins x 10 - losses formula. If I'm not mistaken, it's actually the third from the right (the reason why you had to pick the hardest difficulty!) And also, doesn't the bonus menu only come up in UMK3? It's been a while.(posted 9240 days ago)To throw my two cents in, it's a very brainless effort to play MK3 for hours on end like that. Any halfway decent player with knowledge of the CPU's flaws could only mess up their streak by screwing up the mercy in round 3 against Ermac. I don't think there's any need to change the scoring system at this moment.
Cheers, Brian McLean