Retrieval match is available in any level.
The main goal of retrieval is to collect the objective in the center of the map and then bring it to the other side of the map and score it in the designated spot. There are 2 teams and both are going for the objective. The best way to take down this achievement is to start off by camping wherever the objective is located and allow an enemy to pick it up and then kill them.
After killing them grab the objective and try to score it. You don’t need to score it on your first try as long as you get it into the goal zone. You could of course always grab a buddy, allow him to pick up the objective, kill him, and score it.