Note: It is possible to destroy the first infantry targets WITHOUT using the mortar to get the achievement.
Load up the mission and battle you're way to the mortar tower, But make sure to keep the sniper you start with. Before you go onto the mortar pull out the M14 EBR and pick off the ground forces. From there take out the first convoy that comes, shoot the driver first and then the gunner. Then move onto the Mortar, Move it left as Price calls out "2 more convoys and ground patrol". as the first convoy comes round the corner line up for the shot with the mortar and fire as it stops, this should take out both convoys and the foot patrols.
From there, take it right, and use the remaining 3 mortars to take out the Foot patrol that comes from the same direction as the first convoy. Aim for the convoy and wait for the foot patrol to be next to it and fire, this will blow up the convoy causing a bigger explosion and makes it possible to take out the full patrol with 1 mortar.
hope this helps, remember that you only have 4 mortar's to do this sequence even thought you actually get more, so only use the 4. I did it with only 2 so i hope this helps