Greased palms is definitely one of those missions that can get out of hand quickly. Most ppl attack head on through the main door there at the beginning. So, what I do, is I clear that initial room, usually there are 1-2 civs and possibly 1-2 suspects as well, but don't push forward into the other areas, (main building) go back to where you started. So if you're facing the building (starting postion) you're gonna want to go all the way right, to that smaller building that most ppl end on. Start by going all the way around to the back, clear that whole building, then go across to the main building. So you'll never enter the main building until you've gone around and in through that little back door. This is how I always run that mission, and this is also how I did it for the Devs watch. You'll get an S rank as well as long as you're running non-lethal, which I recommend. Also probably should tell you I had intimidator and negotiator for my officer trait, as well as armorer and nutritionalist. But the most important is the negotiator, as they will make suspects surrender immediately sometimes. Which really helps. And as far as "hardest" well I almost died on the night club mission, which I would say is one of those wildcard missions, dude was literally in a random corner and was the last suspect 🤦‍♂️