Elite has a fine guide, I am going to talk more about the glitchiness of this achievement.
First, the setup:
- 3 skulls: famine, fog and iron.
- I had competitive scoring ON (some think it glitches the achievement when on).
- I did NOT skip any cutscenes.
With this setup, I skipped a few battles at the beginning (including knights) and still finished with 69,000. Also, use the "save/quit" feature then "resume" so you don't restart the entire level with the iron skull on.
Now, the reason I skipped a few battles was because I tried to beat the par TIME while going for this achievement as well. As with other levels, I noticed the achievement had a better chance of unlocking when I hit both par score and par time together.
Again, this was the same with some other levels...I tried Midnight well over ten times with different variations (skipping cutscenes, not skipping them, with competitive scoring on, with scoring on, with no scoring on, on legendary and heroic) I tried everything and none of these helped though I was beating the par time; stats never updated either.
Once I beat the par time and score together, it unlocked. I'd be happy to hear other's thoughts, but this was my experience. Good luck and you might be able to do this one normal. That should help with your speed running.