Edit as 2012/05/29
While some people may play for fun or are trying to find a way to boost, prepare to play for awhile. For your information you will need to get 619,500 EXP to reach Level 50 and we are looking for more than 80-100 hours if you are playing legit. Less if someone find a really good way to boost or if you play/boost during Double XP event.
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-Playing Online to reach Level 50 : 80-100 Hours (See below for information)
-Playing Score Attack Trick 30-40 Hours (See below for information)
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First of all, here's how much EXP you will need pass the Rank 25 to level 50. (As you can see, once you reach around level 34, you are halfway to level 50)
Rank 25 - 179,900 EXP
Rank 30 - 249,500 EXP
Rank 35 - 328,300 EXP
Rank 40 - 416,200 EXP
Rank 45 - 512,700 EXP
Rank 50 - 619,500 EXP
New XP values:
Kill: 50 XP
Headshot : 75 XP
L2 Burst: 25 XP
L3 Burst: 50 XP
Territory First In: 25 XP
Delivery/Grab Pickup: 25 XP
Delivery/Grab Drop-Off: 50 XP
New cash values:
x3 Killstreak: $50
x5 Killstreak: $75
Delivery Pickup: $25
Delivery Drop-Off: $25
Delivery Intercept: $10
Grab Pickup: $50
Grab Drop-Off: $50
Bomb Defused/Planted: $125
Warfare Winning Team: $100
PK Loot Special Corpse: $100 (reduction from $250)
If you wanna know how much Experience you've got so far, go to this link :
http://socialclub.rockstargames.com/ and choose the Games > Max Payne 3 > Multiplayer > Career
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(Solo) Score Attack 15,000-20,000 XP Per Hour Method :
There's a method to rank up while you are playing the campaign, you will have to beat the Campaign once to unlock the Score Attack Option
(Main Menu > Arcade > Score Attack)
Now choose Chapter 4 Part 1, You can choose Free Aim/Soft Aim, it doesn't matter,so you will have to do and finish this level as fast as possible and you will need to do a Platinum Medal to receive 1000 XP. (Gold Medal give you 750 XP)
*CREDITS TO Hawknz from PS3Trophies.org*
When you begin the chapter, shoot the first guy you are prompted to. Then immediately skip the following cutscene. Now just make your way through the bar as quickly as possible, scoring headshots, getting melee kills, whatever you need to do to get through fast. Make sure to pick up a second pistol on your way through and switch to dual wielding ASAP.
Once you're downstairs, continue eliminating the enemies quickly and pick up the 2 PainPills on the bar. You are going for speed here so be reckless, take damage, and eat pills.
Once you exit the backdoor another cutscene will begin. You can skip it once Max is about halfway up the stairs. Once it's over you will be crouched behind cover near two cars with a bunch of enemies standing around. Shoot the blue car first and wait for it to burn and explode, killing some of the enemies. Once it has, shoot the white car second to finish off the rest. If there are any leftover, shoot them and continue on towards the final alleyway.
Once there another cutscene will begin. Usually you can skip this one too, but sometimes you can't if the game is still loading. When you're back in control you will be looking down the alley way full of enemies. Shoot the black car closest to you first. Once it starts burning immediately begin shooting the last two cars that have just pulled up further down the alley. Once they all explode, everyone should be dead and you should now have enough time on your clock to score a Platinum medal and a quick and easy 1000xp.
You can go ahead and click the retry button to start your next run. Don't worry, the XP you've just earned will be saved and you can skip an extra load screen which will save you a lot of time in the long run.
By the way, if you reach Level 50 and you didn't get you achievement while you are still doing Score Attack, go back to multiplayer and join a multiplayer match and you should get your achievement instantly.
OR You can follow a video from Markyshizzle here :
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(Online) Legit Way to Rank Up :
First of all, enter a crew, click the link above and Sign in or Sign Up for the Rockstar's Social Club to access the crews members. Once you joined a crew, join any crews you want.
Q : Why is this necessary to join a crews ?
A: When you are playing Gang Wars/Team Deathmatch, if you are playing against a crew on the other team,you will earn Bonus EXP for saving your teammates (crew members), winning against other crews , etc.
Q : Which game mode(s) should I play ?
A : I recommanded you to play Gang Wars, I usually do 1,500-3,000 EXP during a match.Kill everyone you see & rush for the objectives. You can also play Payne Killer, if you are really good to kill and be Max Payne most of the time you will earn alot of EXP.
Q: Is there a faster way to earn Experience ?
A : Grind. Simply, do every grind you can do : Kills/Headshots with each Weapons. Use each item 250/500 times , Play each Games Modes, Bullet Dodge/melee ennemies, etc. For example, if you complete a weapon, you will earn 3000 EXP (for 1000 kills) + almost 2000 EXP (for 100 headshots) so you are looking for 5,000 EXP for each weapon.
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(Online) Boosting way to Rank Up :
For now, we have the Chargers Sucks method which is playing a Deathmatch with a full party of 8 players and Kill everyone as fast as possible and put a vendetta on the person that keeps killing them. Going for heashots get you more EXP (75+) He recommended the Branco Headquaters map. Thanks & credits to Chargers Sucks.
If someone find a faster/better method , feel free to post one or comment here.
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(Online) Turbo Controller way to Rank Up :
I have tried for the past few days to find a solution to boost myself while I was sleeping/out of the house and found that if you are playing Payne Killer and let you Turbo Controller on ''A'' , you will finish last and earn 150 EXP every 10 minutes. yes, I know it's not alot but for now it's seems pretty good if you are doing something else. For example, if you sleeping for 8 hours , it's 6,000 EXP a day and if you're doing this a whole week , it's 42,000 EXP. This is only for the people who wants to level up as fast as possible and wants to risk leaving theirs xbox open for a long time. At your own risk.
*If you don't have a turbo controller and you don't move for 3-5 minutes the game will kick you *
I will update this guide as soon as I gets more info, hope it helps.