I've beaten MK1 twice now;
1st time - on very easy, lots o' continues, switched fighters - achievement didn't pop
2nd time - on normal, lots o' continues, didn't switch fighters- achievement popped
I hear people have gotten it to pop on very easy. this is pure speculation but I don't think there's any set thing that is pops the achievement, it's just a random glitch due to sloppy programing.
I put my 2 cents into NetherRealm Studios, not so much about the glitched achievement, most players won't care about that, but about the horribly laggy multiplayer and rushed job on this game overall. They may not take any feedback seriously, but feh if a bunch of people complain they may care?