As people may start to notice, Ubisoft has effectively fixed this glitch and is now unlocking appropriately for everyone. If you had already completed this mode, simply complete any sequence again and it will unlock for you.
If you are new to this mode, Discovery mode is basically the tutorial for Wolfpack mode, and can be accessed at anytime through the Wolfpack Multiplayer menu, however the first time you start up Wolfpack it will ask you if you want to do it, so it can't be missed. Simply complete the 8 tutorial levels and the achievement will unlock at the end of the 8th. Happy hunting!