This achievement SHOULD unlock when you pass 777 grimoire score. You don't have to land exactly on it, just have more than 777.
You earn grimoire score by completing tasks of all kinds (story missions, total kill counts with various methods, etc). Basically just by playing. I reached 785 grimoire score after about 15 hours of playing and that included a lot of wasted time.
Currently achievements (all of them) in Destiny are VERY glitchy and may not unlock when they should. If you think you've earned ANY achievement, to include this one, follow the below method to re-sync with the server and get previously earned achievements to unlock. Thanks to Scorch for bringing us the info from XBA.
Scorch - Log in to game. Go Settings. Under Logout, the below option is Change Profile. Click that and it forces you back to title screen. Select your Gamertag then Choose your Guardian. When it loads into the game they should unlock.