If you're anything like me, and you get sick of the multiplayer very easily (if you've played the multiplayer since Brotherhood, you'll know what I mean), I've got a surefire way for you to get this achievement without too much effort!
Seriously though, it's super simple: play single-player Wolfpack for 30mins each day until you get to lv55. You get a Daily Boost for 20mins each day, which doubles your session score at the end of a game, and the challenges on Wolfpack are extremely easy to get, especially if you're playing by yourself. I found myself getting to lv30 within no time, and after that I was levelling up 2-3 levels each session with the Daily Boost.
On that note, the reason I played single-player Wolfpack as opposed to multiplayer was because there are some "strange" people that either don't know how to play the gametype properly, or they just don't care. They put your score at risk by revealing themselves to your targets, and pretty much ensure that you don't make it to the end of the game. Also, it's very possible that you can join games halfway through, which means you end up with a lower session score, and you essentially waste precious Daily Boost time. With single-player, no-one can interrupt you while you're destroying those damn simulations, and you're pretty much guaranteed an amazing score each session.
If you're actually going to follow this guide, try completing every map on Standard difficulty on Wolfpack Unleashed. Doing so will unlock Difficult and Hardcore difficulties, which have an increased number of sequences to complete (i.e. potential for a higher session score). I was pretty laid back on getting this achievement, but if you're dead keen on getting it ASAP, completing the sessions without the Daily Boost still give you a heap of XP, especially with challenges.EDIT:
I've been asked to write a guide with regards to ability sets. Just note that the ability sets you use are completely dependent on how you play the game - I generally play with caution and patience so that I can rack up a higher score for each kill, but other people might have different tactics (like rushing their targets in order to fit more kills into the session). In other words, you should try make an ability set that fits your gameplay style, but I'll share my main ability set for Wolfpack regardless. I haven't played in months, so i'm going to have to list the set off the top of my head. Also keep in mind that I had levelled up a fair bit by this stage, so i'll put some alternatives in there.Wolfpack Ability Set 1
Ability 1: Poison
- This one is (and will probably always be) my favourite ability. You poison your target, wait for the poison to kick in and you get an extra +200 points on top of your kill. This nets you a hell of a lot of points if you get a perfect approach/complete any challenges in the process.
Ability 2: Money Bomb
- The only ability that I really used in Wolfpack was Poison. I rarely used Money Bomb, but on the rare occasions that I did, it definitely helped. If you can't tell which person is your target, you pretty much throw money at the group of people containing your target, and whoever doesn't bend down to pick it up is your guy. Pretty useful if you're unsure who your target is.
Ranged Weapon: Poison Dart
- Pretty self explanatory. It's a stealthy way to kill your target from afar, and is essential if you ever want to get the "Kill your next target with a Ranged Weapon" challenge during Wolfpack.
Perk 1: Hot Pursuit
- In the event that my target managed to get away, this perk was really useful for racking up a somewhat notable amount of points. It slows down the depletion of your Approach meter and increases your speed when you're chasing a target.
Perk 2: Resistance
- If you play Wolfpack enough, you'll know how agonising it is to be stunned. Resistance decreases the amount of time you spend suffering from the stun.
Kill Streaks and Loss Streaks are more irrelevant in Wolfpack than standard online play. It doesn't really matter what you use, just whatever you think works for you. A lot of perks and abilities aren't viable for use in Wolfpack because many of the elements present in standard play (*cough* dying *cough*) aren't present in Wolfpack, and as many perks and abilities require these elements for efficient use, they're not really warranted.
Alternative abilities include Firecrackers and Glimmer. Firecrackers are essentially the same as Money Bomb except they blind everyone as opposed to throwing money at their feet. Glimmer makes you "invisible" for a time, giving you a chance to sneak up on your opponent. An alternative perk that I'd use would be Blender, which morphs a civilian into your lookalike when you enter a crowd. This is useful if you're tailing someone in a crowd, but besides that it doesn't do much good. Also, using a Pistol as opposed to a Poison Dart as your Ranged Weapon is just as efficient, but it nets you less points. With the exception of Poison and Poison Dart, most of the perks/abilities that I used can be unlocked relatively early, so once you find a set that you're efficient with, abuse the hell out of it.
And if you think this achievement is tiresome, you should've tried the one on Brotherhood...