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.
To access the toy's location, you must acquire a Spectre. This is most easily accomplished by taking over the Spectre at the beginning of the level. Leave it at the base of the cliff as you enter the structure. Once you meet up with Johnson, jump back down from the platform to where you left the Spectre. Drive the Spectre all the way through the level to the water near the entrance to the control room. Drive through the water to the very right of the map, where a land bridge juts up out of the water and curves back toward the control room. Follow this land bridge and use the Spectre to climb to the very top of the mountain behind the structure. The final toy sits on the summit. It is highly advisable to use Classic graphics mode when using the Spectre to climb the steep cliffs, as it shows the true geometry of the level.