This is completable just after you finish the "Fireworks" mission.
Make sure you grab the tomatoes (at least 1) from the building that you're climbing to place fireworks at the top. The tomatoes are located in the area where there's a group of survivors. (Next to a set of windows on a long wooden desk). After the mission, when you're on the streets again, go to the uppermost building on the block above the shopping mall
There's a ladder leading directly up to the roof of this building. There should be 1 enemy with a cleaver at the top. Dispatch him and then go up the ramp/stairs on your left (after climbing to the roof where the enemy is). In the left corner furthest from where the ramp leads you is a mother and son. The mother is in need of 1 tomato. Give it to here and the achievement will pop. :)