Luckily, I'm one of the 5% of users who gotten "Monument to all of your sins" and happy to share the experience. If I were to sum it all down to two sentences, it would be "AHHHHHHHHHHHHHHHHHHHHHHHHH WHAT THE HECK!?!?! HOW DID HE KILL ME THIS TIME?!?!" Yeah, it wasn't a pleasant experience. The hardest part was at the end ((SPOILERS)) where you have to fight waves of enemies just before you get picked up by captain keyes. It was so brutal. It's so easy to run out of ammo so darn fast. But after that, I only had to go through so many zealots with over shields which was a challenge also. But after that, I felt very accomplished. And I only need one more achievement which is "Make it rain." It was very hard though. Tougher than halo 3 because the AI mechanics for the enemies are smart as hell. While the AI for noble six/marines, is smarter than your average brick. My total playtime after it was about 15 hours.