Final dungeon is not the Sealed temple, but the floating gardens that appears after you have beaten every dungeon on King's Road. It's basicly 50 floors of enemies and bosses on King's road difficulty and you can't use healing items. You can use items that cure poison, bleeding etc but not the ones that give you hp or mp. Only hp item you can use is fortune coin which has 50% chance to give you life or take some of your life but it won't actually kill you.
In this game you have res stones that bring you back if you die BUT the game only gives you 10 of them and if you wan't more you have to buy them with microsoft points. The reason I say this, is because this is the hardest achievement to do and you should save all the res stone you can for this. It's still possible to get 400 from this game without buying more res stones. Check the Roadmap at the start of the guide for this!
Note: If you fail to meet the time even by 1 sec, you can't get back those res stones you used (quit to dashboard doesn't work). So be prepaired!
Use the Achievement Trading Thread to find an online party if needed.