When you beat the game and save after the credits. Just reload from that save. You won't have to go through the game again in like 3-5 hours to re-open all the areas. The only thing you'll miss is some treasure chests that have some of the GR7 armor/weapons.

I did it today while no one was online, but I spent a day or so questing the II quests. Let me tell you, the game is easy, and then gets challenging in 2.5 seconds.