Complete Level 4Story-related and cannot be missed.
To level up, you must complete Campaign Missions, as they're the only source of XP. You'll reach Level 4 and earn this trophy when you complete your fourth Campaign level. As you continue to do missions, more Campaign Missions appear on the map. You can do these in any order you please and you'll always gain XP no matter which one you do.
See Roll Credits for more information and tips.