Here are some cheat codes that will turn you into a ninja while you're on the Gamertag Nation site.
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MarvzMitts posted the following guide:
To unlock this achievement you will need to complete a daunting list of tasks found in Liberty City. This achievement will take some time to complete, here are all the requirements needed to unlock the achievement:
1) Complete all 94 Story Missions & Don't kill anyone when given a choice (This is really important).
2) Complete all 10 Drug Delivery Missions for Little Jacob.
3) Deliver all 10 Exotic Export Cars to Bruice's garage from his email requests.
4) Win all 9 street races given by Bruice.
5) Deliver all 30 cars to Stevie's garage from his text message requests.
6) Use the computer in police cars to kill all 10 of the most wanted on each island. There are a total of 30.
7) Use the computer in police cars to complete 20 crimes in progress.
8) Unlock Little Jacob's gun delivery ability by raising his like status above 75%.
9) Unlock Brucie's helicopter ability by raising his like status above 75%.
10) Unlock Packie's car bomb ability by raising his like status above 75%.
11) Win a game of pool against any of your friends.
12) Win a game of darts against any of your friends.
13) Win a full game of bowling against any of your friends.
14) Beat the high score of 11,000 on the arcade game QUB3D.
15) Complete all 6 activities with Little Jacob: Darts, Drinking, Eating, Pool, Show, & Strip Club.
16) Complete all 6 activities with Bruice: Darts, Drinking, Eating, Pool, Show, & Strip Club.
17) Complete all 6 activities with Packie: Darts, Drinking, Eating, Pool, Show, & Strip Club.
18) Meet all of the random strangers and complete all of their side missions. Use this solution - http://www.gamertagnation.com/achiev...ievement-1854/
19) Complete all 9 assassination missions from the payphone in Alderney.
20) Find and kill all 200 pigeons scattered around Liberty City.
21) Find and successfully complete all 50 stunt jumps scattered around Liberty City.