WATCH_DOGS

There are a total of 57 skills to unlock in Aiden’s Skill Tree (Online Skills don’t count). To purchase a Skill you'll need Skill Points which can be obtained by levelling up Aiden, which is done by completing story missions and Acts, taking part in the multiple activities Chicago has to offer, or simply finding collectables.For two Skills you need to complete City Games (NVZN to Level 5 and any Chess Puzzle to Level 10)

here is the Solution for the End Game Puzzle in Brewed (The Loop)






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Posted by PSYCHO POOCH: do not need to play chess till level 10...you only need to play 10 chess puzzles. Which could be 5 different ones to level 2. I played 3 types of chess puzzle and every one i did added to the counter

Posted by elKamikazeSqui: ... Should also mention that Gang Hideouts reward you with Skill Points for finishing them too.

Posted by Famous Yeti : For anyone that is short on Skill points: Go to Options > Gameplay > Reset Side Mission Content. You keep all your progress but you can redo the side missions (like Gang hideouts) to get more Skill points.

Like King FishiZ said, there a total of 57 skills to unlock. Getting xp to earn skill points is relatively easy as simply doing nearly anything earns xp, so there is no main problem there. IF you are having trouble, there are two progression awards that give you general xp bonuses. One is rewarded for reaching level 3 in any one of the city's Shell Games. The other for collecting 10 Audio Logs.
HOWEVER, you may find that when you earn xp, you are no longer leveling up. This is because the level cap is below the required amount of points to unlock every skill. This is solved by completing ALL Gang Hideouts, which give you 2 skill points per hideout (there is a total of 15, giving 30 skill points). There is also an achievement for completing all gang hideouts, so anyone going for 100% should have no problem. I ended up with one skill point left over as surplus.

The other issue is unlocking all the skill upgrades. By completing act one, mission 5 you unlock "second tier" upgrades, meaning it is at this point you can unlock any of the upgrades.
Here is a list of the remaining locked skills with tasks required to unlock them:
1) Critical Focus - Complete 1 Criminal Convoy Missions
2) Quick Switch - Catch 1 Criminal with ctOS crime prediction
3) Maximized Focus - Complete 10 Chess Challenges
4) Auto Weapon Expert - Reach Level 5 in NVZM AR city game
5) Rapid Reload - Complete 5 Gang Hideout Missions
6) Offensive Driver - Complete 5 Fixer Contracts

You shouldn't have too much trouble with these. But i will mention "complete 10 chess challenges" is NOT reach level 10 in a chess challenge. I found a group of 4 chess tables in the north east of Mad Mile. The lower level challenges are much easier, so i suggest alternating between these 4 tables.

When you come across a gang hideout (fist on the map) you can finish it then go to options and you can reset the hideouts. This method is fast and gives you enough xp to level up every time.