Chance are that the most obvious left-over for you to work on is going to be play-styles. Review the following Play Style list and compare that to your in-game listing to see what you have left to do, and then complete the ones you have left undone...
Play Style List and Hints
Take three hostages (human shields) and knock them out (no kills).
This one is a bit more difficult since you cannot allow ANY civilian deaths at all for this to unlock. The easiest level to unlock it is the King of Chinatown, as you can trigger an alarm condition using the Cop in the alley leading to the Dealer's flat, and then simply grab civilians near the mouth, pull them in, and then knock them out.
Complete five kills with beverage bottles.
Welcome to Hope is a good one for this, or the Terminus Hotel, as both have no shortage of bottles or targets that you can attack with them.
Complete five kills with a Fire Axe or a Tomahawk.
The easiest level for this is Welcome to Hope. Start the bar fight, head to where the Bartender is, but instead of talking to him, take the Tomahawk from the dart board and start killing civilians.
Complete five stealth kills with a knife.
Simply load up King of Chinatown, grab a knife from one of the market stalls and start killing civilians with it until this unlocks!
Complete five kills with bare hands.
Any level with a lot of civilians will work, but King of Chinatown is probably the most convenient. Just have nothing equipped and start attacking them until this unlocks.
Complete five kills with Remote Explosives.
Any level with Remote Explosives will work for this – simply grab the pack toss it into a crowd and Boom! Mischief Managed!
Complete nine kills with a Katana.
During the mission Hunter and Hunted you will have access to a Katana - simply take it and start killing civilians in the crowd.
Complete five kills with a shotgun without being damaged.
Any level you can get a shotgun in will work, you just have to manage five kills without taking any damage yourself. You may find that this is easier on Welcome to Hope since you can get the Shotgun from the office via the heating vent in the bathroom, start the bar fight, and then just pop off a couple rounds to unlock it.
Complete fifteen kills with a revolver.
One of the best levels for this is the courthouse level in Skurky's Law, since a LOT of the Guards have revolvers. Take one and load up on ammo then start shooting!
Complete two accidental kills.
This will unlock as part of regular game play.
Complete five headshot kills in a row with a sniper rifle without missing.
Probably the best place to do this is when you do the bridge shots on the Dexter Industries level. You can easily do five undetected head shots in a row with the sniper rifle from the platform up the stairs.
Complete five stealth-kills with a syringe.
This is best completed on the Rosewood - Orphanage Halls level, since there are several places with Syringes. You can do the kills as melee kills from behind or as throws, bearing in mind that when you throw the syringe you will need to reclaim it...
Complete five Fiber-wire Garrote Kills.
This is another of the Play Styles you should have unlocked as a normal part of game play... If not, load up a level like the Cornfield in The Saints and go to town with the Garrote!
Complete five sharp-object throw kills.
To make this very easy on yourself load up the Cornfield level of The Saints, kill the guy at the start by the truck in the road, and take the Agency Tanto Knife off of the crate there. Now simply go into the cornfield and start lobbing the knife into Grunt heads and it will quickly unlock!
Complete eight Point-Shooting Kills.
This is another of the Play Styles that you should have unlocked during normal play. If you have not, then simply load up a level and do some Point-and-Shoot.
Complete three Choke-hold and regular subdues.
If you have not unlocked this as a matter of regular play, simply load the Cornfield level for The Saints and have nothing actively equipped when you take down the Grunts in the cornfield and you should have this easily managed.
Complete three kills with a silenced handgun.
This should unlock as part of normal play, but if you do not for some reason unlock it that way, load up the Cornfield level of The Saints, then arm the single Silverballer with silencer, and do stealth kills on three enemy to unlock it.
Take damage twenty times from enemy attacks.
This will unlock as part of normal game play.
Hide five pacified bodies in containers.
You very likely already unlocked this since it is awarded for simply hiding the bodies of people you have incapacitated in containers. If you have not for some strange reason, load any level and start incapping and hiding bodies.
Complete five kills with wooden clubs (baseball bats, golf clubs, sledge hammer, etc.).
Load any level for this, but if you need one to load, Welcome to Hope works good as there is a bat in the bathroom. Grab that, start he fight and then whale away on civilians until this unlocks!
Lastly, if you played the game through on Normal Difficulty – which is really the sane approach if you ask me – there will be just one Achievement left locked, and that is the “Absolution” (50G) You completed Hitman: Absolution as a professional.
Let's get something straight about this: you do not have to start a new game play session. In fact all that you really need to do is replay each level in each chapter with the difficulty set to Hard or higher (Hard or higher being what is defined as professional difficulty).
You need to bear in mind that the game is very different on Hard and incredibly difficult on the higher level. So you will need to work at it if you really want to unlock this one, but not as much if you do it on Hard rather than the higher difficulty. A few tips to bear in mind are to try not to rely upon Instinct to keep you out of trouble, to remember that on Hard and higher they never stop calling for backup, and the point – even during a Rambo-style run-through – is not to kill them all, but to complete the objectives and exit the zone. That pretty much covers that!