For this achievement you must complete 53 of the 56 ambient freeplay events in Saarbrucken. Saarbrucken is the the very top right section of the map, this section will never regain color as it is in Germany.
There is only one entrance and that is through a huge gate, you will need to push the button (with Y) to open the gate and get inside.
I suggest leaving this until last, but its up to you.
Be careful up here with your explosives, make sure you have max Dynamite and RX as well as the Panzershrek rocket launcher with you. This is because there is no weapon dealer or headquarters nearby, so if you die or run out of explosives you will have to travel a good distance for more or if you die.
There are a lot of car jump events in Saarbrucken, so make you bring some sort of race car with you, as they don't give you much starting room so you will need a fast car to make some of the jumps.
Also its best to have the gold perk on Demolitions, as this will let you be able to sabotage while disguised without raising alarm. Doing this will allow you to bypass a lot of fighting by letting you bomb anything as long as you are disguised.
Overall as long as you don't need a return trip this should only take 45-60 minutes.