Hardline difficulty is unlocked once you beat the game in officer or veteran mode. The enemy's weapons will do more damage, and they will have more health in this mode, but none of that really matters if they don't see you.
Hardline mode isn't that hard to hammer through, and you can probably knock it out in a day. Just sneak, sneak, and sneak. Most levels you can avoid about 70% of the enemies, just by walking wide around them, or distracting them. For example, in the last chapter, you can swim in the water around the left from where you start, and avoid the entire island. Just swim until you get to the two guards near the grappling hook area by the mansion, knock out the two guards, grapple up, knock out the guard at the top, head left through the bushes, over the waterfall, knock out the dude by the door and the game is over.
For those moments where sneaking is not an option, just grab some guns you're comfortable with, a gas mask, and get comfy in a corner somewhere and funnel the enemies in a hallway or something like that. They're not very smart, and even on hardline mode, they will still just line up outside a door way and come at you one (sometimes two) at a time. Super breezy.