You can actually get this achievement by only using 3 mortar rounds and destroying 3 targets.
As you climb the tower instead of jumping straight on the mortar, use your sniper rifle to pick off the 4 or so soldiers on the ground and both the gunner and driver from the first technical. Once these two guys are down this technical is no longer designated a target when you use the mortar,
Then jump on the mortar (this triggers the arrival of the remaining targets). You should then have 4 rounds left to destroy 3 targets (I only took 3, but would assume the 4th would be available to use if you so needed it).