Not terribly difficult to do.
you need to beat veteran difficulty to unlock hardline. While playing on veteran try to find all the evidence and get as many arrests warrants as you can. To make your job much easier i recommend the following load out Heavy pistol the m911 or the desert eagle are both acceptable choices. As for a primary since you will do a fair amount of shooting and killing, while trying to remain stealthy wont always be a option. The M240B machine gun is the best pick due to its rate of fire, stopping power and sheer amount of ammo you carry with it makes it ideal. Tactical slots/perks all you really need are the armor vest and stun gun. your ai will usually throw a med pack for you ammo is never a really a issue depending on your play style. Just play smart take your time use cover, when ever possible (you will need it) and the more enemies you can knock out, via stealth and stun gun while avoiding unnecessary gun fights will make things easier.