Cargo is a capture the flag type match where each team has a base with cargo to be taken. This achievement requires you to kill the enemy
who is carrying your team's cargo after stealing it. Once they are carrying the cargo they show with the cargo icon over them.
You could either camp out your base and kill them after they steal it, find them and tak them out on the move or wait at their base and kill them when they try to capture the cargo though that one is the hardest.
Very unlikely you'll get this in one match which is good since this is cumulative.
Mince Pie 77 notes the Infantry Turret is good to setup at the Cargo spawn.