This will not only give you more experience but more importantly it turns all of the neighbourhoods on the map friendly. It is advisable to complete a good amount of the main campaign missions as possible before starting the Activities.
If you're alone and still wish to make a start concentrate on the collectibles, the main campaign, unlocking shortcuts etc and getting a good feel for the gameplay until your Co-op buddy is with you. DO NOT even go near any of the main 12 Activities without your Co-op partner. To ensure this achievement doesn't glitch you must start a brand new campaign, on easy if you like, and complete the first few tutorial missions. Some important points before we systematically complete these activities: You must only activate them when you are connected and ready with your co-op partner. It is essential to start a new game and not to touch any of the main 12 Activities in single player.
IMPORTANT: The achievement for completing all Activities in co-op 'Confidence Men', cannot be unlocked if you have completed any level of any Activity alone.
