Play with whatever skulls you want, on whatever difficulty you want! Only the final score matters.
You MUST provide a Bungie.net Carnage Report (and film) as verification.
An excellent example of a High Score Speed Run: 401,679 points on Crow’s Nest.http://mythicafiles.bungie.org/halo/highimpacthalo/highspeed/Halo3/HighScore/Crow_Pahis_401679.mp4
What games can I do High Score runs for?
Only the ones with a ‘scoring’ feature. These are Halo 3, Halo 3 ODST and Halo: Reach.