The Saboteur

There are 10 different types of perks each with 3 stages.
For this achievement you have to achieve a level 3(gold) in one of the ten.

This can take a bit of time and some are harder than others to achieve.

Alternatively you can buy the upgrade from the shop for 3000 and upgrade any of your silver perks to a gold level and unlock the achievement like this.
If you have no silver perks, theres an upgrade for 2000 to upgrade 1 perk to silver level.

Another relatively easy way to gain a Gold Perk in focusing on the Sniping which doesn't require as much as other perks.

Firstly, purchase a Scoped Carbine from the Weapons Dealer for 500 contraband and have it maxed with ammo.

For the Bronze Perk, you will need to kill just 5 Nazis with this weapon through the sight and it can be anywhere on the body.

For the Silver Perk, you will need to score 15 headshots on Nazis. This initially seems tricky because before you claim the Bronze Perk the aim shakes quite frequently. Fortunately, this is negated once the Bronze Perk is unlocked making this a lot easier.

For the Gold Perk, you will to achieve 10 'double kills' with the scoped weapon which is basically 2 kills for 1 shot. Initially this sounds horrendous but there is a way to achieve this very easily with just a bit of careful judgement.

Once you exit the Belle, immediately turn left and you should come across a guarded area with two guards outside. Before the area on the pavement there are bollards, just before the fourth bollard you can get a perfect shot through both guards any closer and you will be spotted.

Return to the Belle immediately after making the shot and simply enter and exit, this makes those two guards respawn and you can repeat 9 times to unlock the Gold Perk and get the 'Gold Medalist' achievement.

(Credit goes to cra on www.x360a.org for the very effective Gold Perk strategy)

Any questions, feel free to message me and I can help. Happy hunting :)

One (relatively) quick way to earn this one is to hijack an AA gun - one of the smaller rapid fire ones - and just go to town on any nazis that turn up.

Edit: forgot to mention that this works best with the ground-based guns scattered around the countryside (Champagne-Ardennes for example). The roof top ones in Paris can't depress low enough to get the job done.

Do it for long enough and you should be able to notch up the Mayhem, Hardware, and if you get the alarm level high enough Evasion gold level unlocks. Took me about five, maybe ten minutes.