From Halopedia, the Halo wiki
It is represented by a red and silver signpost with a Rocket Launcher in front of it.
- The easiest way to unlock the achievement is give a rocket launcher to one of the marines riding on the treads of your Scorpion tank. The marine will have infinite ammo, and every kill they score while riding on the tank will count toward this achievement.
- If you need additional rockets to give to a marine, or you just want to get the kills yourself, there are additional rocket launchers throughout the level. The first, containing four rockets, can be found at the very beginning of the level, behind a barrier behind the second Scorpion tank. An additional rocket launcher can be found in the final area before you meet up with Dutch. Instead of going straight to meet Dutch, turn left and follow the street back in the opposite direction. Behind a crate is another rocket launcher with four more rockets, bringing the total to ten. If this is still not enough, a final rocket launcher is held by a marine who is standing next to the slagged Gauss turret which you found to start the level.