And So It Begins...
From Halopedia, the Halo wiki
The toy only appears in Remastered graphics mode. The achievement is worth 5 Gamerscore and can be unlocked on any difficulty.
Upon reaching the bridge, grab one of the Ghosts and follow the path back in the direction you came. When you reach the area of the path where you encountered the Jackal snipers, boost up the cliff wall. It is strongly recommended that you do this in Classic Graphics mode as this will show you the true level geometry on which the player must drive. Boost your way all along the ridge which rings around the lake until you reach the end of the rocky peninsula. Change to Remastered graphics and you will find the Master Chief toy sitting on the ground.