Not many people have earned all the achievements possible from Halo 3, Halo Wars and Halo 3: ODST, as evidenced by the statistics Microsoft dug up while developing Halo Waypoint. "Significantly less" than 1% of players have unlocked all the achievements. Based on readers comments and e-mails G4TV received, the biggest stumbling block for Halo fans is a top-tier achievement tucked away in Halo Wars, the non-Bungie developed real-time-strategy take on the Halo universe.
Specifically, several readers cited the "General" achievement as the most difficult to obtain. The "General" achievement is obtained by acquiring 2,400,000 experience points across multiplayer sessions. According to players, garnering that many points takes a long, long time.