The start of the mission FRONTLINE
is your best bet here. There is a rail car with about 6 soldiers on it. What I did was kill the commander which would trigger the other soldiers to jump out of the cart. This is where I would throw a fire grenade on either side which burned most enemies. After that I would reload checkpoint as it doesn't reset your progress and repeated until I unlocked the achievement.
If you don't have any grenades then you can load up the level right before it called ARMORY
where you can buy them from one of the vendors.
After this achievement you can switch to normal grenades to unlock: