You get this achievement for beating the game, it pops after the credits stop.
Here's a road map on how to beat the game.
You can still do everything after beating the final boss in the Throne of Want (you'll need a soul of a giant before the boss pops up in the Throne though). After beating the final boss, you can hit NG+ whenever you want by clicking the Majula bonfire. I recommend slaughtering every NPC you can for giggles and loot before NG+ The hag ladies you meet in the beginning of the game drop 8 human effigies if you kill them!
I finished the game around level 190 but only because I knocked off all the NG achievements first. Realistically most people finish the game around level 100-120.