007 Legends

Achievement Name :
Signature Worth : 7G
Achievement Objective :
Campaign: Reach level three weapon proficiency with the P99

For this achievement you need to make 105 kills with the Walther P99 to reach level 3 of the XP Challenge (Walther P99 Elimination : III)

You can track your progress by going to the XP Challenge menu.
Main Menu>>Single Player>>Agent Profile>>XP Challenges...its the top challenge under the Weapons tab.

Good weapon to play with when you like to be silent but deadly :D

EDIT 1 : An additional bit of information is that you can restart checkpoints and abuse the constantly spawning enemies at some points.(according to matdan12)
I can't confirm it myself since I don't have the game at the moment.

HARDC0REWARRI0R- Has a really good solution however I have further information that will save time if you don't unlock this while completing the game (I didn't).

First off any kills you get will only count when you hit the next checkpoint and if you die before you do or restart checkpoint you lose any progress you made, at least as of today 17/03/16.

So you need to find a level that you can kill enemies, then hit a checkpoint, then restart the mission and rise and repeat.

So the level I found was - Die Another Day- Graves Cargo Plane- this is one of the few levels without an unskippable long intro. You start with the P99, make your way through the plane killing enemies until Jynx goes up the ladder and you enter a closed door with a light up map screen in front of you, once you enter the room the game checkpoints and you can then restart the mission and repeat this until you have the achievement.

In addition you should be on operative difficulty and select all health upgrades (if you have them unlocked) to make this much easier, you can select upgrades anytime by hitting select and scrolling to training modules after buying them in the upgrade shop.

It's also worth noting this is an odd Gamerscore achievement at 7G which means you might want to actually avoid this achievement if you want to keep your Gamerscore ending in a 5 or a 0 depending on if you have done the other odd achievements in the game these being 16G with or without Q and 12G positively shocking.

Hope this helps.