Watch Dogs

This multiplayer game mode will not unlock until you've completed the first half of the 'Backseat Driver' mission in Act 1. You'll also need the Online Invasions setting set to 'On', for you to be able to take part in this mode. From the main game, bring up your Smartphone and select the 'Online Contracts' App. From here, select 'Online Tailing' then finally select 'Request Tailing Target'.

This mode requires you to simply not act like a hacker at all and more like one of the civilian NPCs. You'll be placed into another players game where you must simply observe them while while your percentage meter rises to 100%. The other player doesn't not know you're in his game, so don't give your position away.

The other player will only notice you if you act suspicious like pulling out your guns or driving crazy. Once they've spotted you, it'll come a lot harder to to complete your hack as they'll either start shooting at you and you'll die, or they'll drive away. So your best bet is to take it nice a slow and stay under the radar.

If by chance you do get spotted and the other player quits out, don't worry. This still counts as a win for you and will go towards your overall total of the achievement. You can view how many you have left if you bring up your Smartphone, select the 'Statistics' App and then press to access the 'Progression' menu.