For me, it's a 10. Not as hard overall as Ninja Gaiden 2, (Which I've also gotten the full 1000 for) but Challenge 6 alone is more difficult than any single part of NG 2, even Chapter 11 on Maser Ninja. It took me about 17 hours to just to beat Challenge 6. 17 hours on one little challenge? Now that's insane.
Full 1000 in:
Ninja Gaiden 2, Vanquish, Lost Planet, Sonic Unleashed, Mass Effect 1-3, Alpha Protocol, Fallout 3, Fallout New Vegas, Elder Scrolls IV: Oblivion, Dragon Age: Origins, Sonic The Hedghog (2006), Sonic Generations, Eternal Sonata, Tomb Raider Legend, Tomb Raider Underworld, Resident Evil 5, Halo Reach, Fight Night Round 3, Sonic's Ultimate Genesis Collection, Modern Warfare 2, Call of Duty 2, Bioshock, Star Wars: TFU, Prince of Persia: TFS, Terminator Salvation, King Kong.