5. Halo 3 - Vidmaster Challenge: Annual
Here's another achievement that was added in later with DLC this time it's the massive Halo 3. Admit it, Halo 3 is one of the main reasons you bought a 360 all those years ago. This achievement is one of the ones needed to unlock the famous Recon armour in multiplayer, making it hugely desirable back in the day. It's interesting then that the achievement offers 0 gamerscore. Over Xbox Live, you and three other people need to beat the last level of the game (imaginatively titled Halo) on Legendary. You also need to have the Iron skull activated (when you die, you return to the last checkpoint instead of respawning) and you need to finish the level with everybody riding Ghosts. It's a challenge and it'll take you and some buddies a while to complete. The hardest you'll ever work for next to nothing? Quite possibly, but it's hugely satisfying to see that 0GS addition pop up in the corner.