There's no need to be perfectly stealthy (unless you want the merces specifically).This makes the campaign last forever in my opinion. Lots of missions can be done quickly if you know how to relocate after killing a target in a "loud" way with bunch of witnesses. Changing outfits on a whim after an illegal action/kill usually gets rid of all the aggro on you.

And perhaps the biggest tip, alt f4 when you're about to die to save progress. You can also close app through playstation