I Am Your Shield
From Halopedia, the Halo wiki
I Am Your Shield is an achievement in Halo: Spartan Strike. It is awarded for keeping the M312 Elephant 100% intact in Operation A, Mission 3. The achievement is worth 5 Gamerscore. It is represented by a circle with an image of an M312 Elephant inside it.
Although some of the enemies on the level only spawn once the player advances forward with the Elephant, many others are spawned on the level from the outset. In order to increase their chances of unlocking the achievement, the player can push forward ahead of the Elephant and eliminate these enemies before doubling back to advance with the Elephant.
The name of the achievement is a reference to a quote by Cortana "I am your shield. I am your sword.", as spoken in both the Halo 3 level The Storm and the Halo 3 announcement trailer.