To me, Combo and mobility are the two key components to get a high score. Time bonus may seem nice, but its impact on the final score is not as significant as the two others I mentioned. Getting the frog mask makes getting a big combo easier. I usually focus on getting the largest combo possible while refraining from too much use of guns, ignoring how many tries it takes me.
In killings, you simply get points for how you killed your enemy. You get the most points from kills if you are unarmed, and less using melee weapons. Fire-arms do not give a lot of points but they are nice to get a big combo. Exotic weaponry such as the drill gives the most.
Boldness should be how reckless you are. If you put yourself in dangerous situations where the enemies try to attack or shoot you, boldness goes up. I am not completely sure about this though. Use of firearms can have a direct negative impact on your boldness score.
Mobility drops if you are staying put all the time, waiting at doors and behind corners. Moving about from room to room without stopping ensures a high mobility score.
Flexibility is supposed to be how you make use of the weapons available to you. If you kill every person on a floor with fists only, you get a high killing score but low flexibility. Constantly changing weapons from what's available improves flexibility. If you are never getting any flexibility, you may need to patch your game.
When you manage to clear an entire floor in a single flowing combo, you are pretty much guaranteed the A+ rating.
Another way to get very high scores is to use non-standard weapons such as the drill which gives 5600 points per kill.
Edit: The points awarded from Drill-kills have been reduced to ~1000 points in the latest version.
I used this video extensively to figure out each of these. Here they are:
- U - Prelude: The Metro. 1st Floor, Bottom Left end of the tracks
- T - Chapter 1: No Talk. 2nd Floor, Top left kitchen in front of sink
- A - Chapter 2: Overdose. 2nd Floor, Bottom Right bathroom (between toilet and tub)
- O - Chapter 3: Decadence. 2nd Floor, Bottom Left dining room in front of chair (furthest right, top row)
- A - Chapter 4: Tension. 2nd Floor, Bombroom (left below window)
- E - Chapter 5: Full House. 1st Floor, Bottom right kitchen (right of top chair)
- I - Chapter 6: Clean Hit. 2nd Floor, Bottom right balcony (below head of lower patio chair)
- N - Chapter 7: Neighbours. 1st Floor, Top Left Bedroom, right of computer chair.
- R - Chapter 8: Push It. Right 2nd Floor, Top right corner of top right dance floor
- S - Chapter 9: Crackdown. 2nd Floor. Top middle by elevator? BE FAST, you have to be the SWAT
- I - Chapter 10: Hot-N-Heavy. (Top right floor), bottom left pool room, below bottom pool chair
Spoilers:
Jake is also found in this chapter, on a dead body in the uppermost room of the top floor
- N - Chapter 11: Deadline. 2nd Floor, Bottom Left
- W - Chapter 12: Trauma. Starting Floor, bottom right bathroom next to toilet.
- S - Chapter 13: Assault. 1st Floor, Bottom Left bathroom (between toilet and "sink" (I think it may be one of those sinks you wash your bum with, but I don't think they're common in Miami? (they're not in Canada...)))
- B - Chapter 14: Vengenace. 3rd Floor, Bathroom in front of stairs (upper left urinal)
- H - Final Chapter: Showdown. 1st floor, room on the left between coach and chair.
This spells out IWASBORNINTHEUSA, which is the password to the computer in Chapter 19. It seems to changed the dialog if you view the computer before you talk to the guys
Best Answer
You must collect every little purple block (as Berg mentioned in their answer) in chapters 1 - 16. I detailed where every piece is in this answer. When you collect them all, you must align them in the "Puzzle". When a piece is in the correct space, it will start glowing. The final result is: