1 - Needle Pumper :
- Kill 5 enemies with a syringe without being spotted. Throwing a syringe works too.
The best chapters are "Rosewood : The Orphanage" and "Death Factory : R&D" as they are the only ones in which you'll find syringes.

2 - Undertaker :
- Hide 5 bodies in wardrobes/containers.

3 - Piano Man :
- Kill 5 enemies using the garrote.

4 - Reaper Man :
- Kill 8 enemies using Point Shooting. You don't have to kill them all at once, you can use Point Shooting several times during the level until you reach 8 victims.
It works with innocent people, if you feel like shooting at a crowd.

5 - Pitcher :
- Kill 5 enemies by throwing sharp weapons without being spotted.
This works with the following : knives, scissors, fire pokers, hammers, screwdrivers, meat cleavers, katanas, syringes, axes.

6 - Sandman :
- Subdue 3 enemies. To do this, you need to be bare hands (holster your current weapon/item), and attack them from behind with the corresponding Attack key that displays (Q or A by default) until they're subdued. /!\ Do not break their neck ! (Z or W)

7 - Bank Robber :
- Knock 3 enemies out while using them as human shields. You need to isolate 3 enemies one by one and knock them out without being seen by others. Use the default X key while sneaking behind them, or if you're spotted being suspicious, enemies will ask you to surrender.

8 - Spook :
- Kill 3 enemies using silent elimination. To perform this, sneak behind them with your silenced silverballers and press the "Action" key. This will kill them without a noise.

9 - Dynasty Warrior :
- Kill 9 enemies with the Katana. It doesn't matter if you're spotted or not.
You will find this weapon in the chapter "Hunter and Hunter : Chinese New Year" and in "Blackwater Park : Penthouse".

10 - Dynamite Harry :
- Kill 5 enemies with explosions. You can use Gas Cans, Proximity Mines, Remote Explosive, C4 bricks ("Fight Night"), Ovens, or the pump station in the "Shaving Lenny" chapter. It can be done with multiple explosions within the same level. I found it easier to achieve on "Dexter Industries : Descent". There's a bridge overhanging a big room with a group of soldiers. If you drop a Proximity mine from above, you can kill up to 6 of them in a single explosion, and you won't even be spotted !

11 - Gunslinger :
- Kill 15 enemies using a revolver. This can be achieved with the Z&M Model 60, the Aries Charging Ram or the Swiss Derringer. Hope cops all have revolvers, it's easier to do this in a corresponding chapter, such as "Skurky's Law" or "Operation Sledgehammer : County Jail". Another recommended level for this is "Terminus : Terminus Hotel". There's a revolver in a small courtyard, and plenty of bad guys waiting for you.

12 - Maestro : Kill 5 enemies with a headshot in a row without being spotted. You can do it easily at the beginning of the chapter "Dexter Industries : Factory Compound", "Operation Sledgehammer : Hope Fair" using the Sniper Rifle and a nearby vantage point.

13 - Glass Cannon : Kill 5 enemies with a shotgun without being hit a single time. You will need to be quick to achieve this before enemies start shooting at you. Take advantage of the slowmotion effect that starts with each shooting.

14 - Bartender : Kill 5 enemies using bottles. This is easily done in the mission "Welcome to Hope" as you can take advantage of the fight to not be in trouble. Bottles are usable only once, but there's plenty to pick up in this level.

15 - Champion : Kill 5 enemies with bare hands ! You can't kill them from behind, you must go through the QTE interaction.
Same as before, you can do it easily in "Welcome to Hope".

16 - Viking : Kill 5 enemies using clubs. This works with the baseball bat, the golf club (found in "Skurky's Law : Courthouse") the sledgehammer or the mace.
The hammer doesn't count!.
You need to be in front of the enemy for this to work : if you sneak behind, Agent 47 will strangle them instead.

17 - Brutus : Kill 5 enemies with a knife from behind, without being spotted. You can't throw the knives, you need to slice their throats to achieve this Play Style. This can be done with the kitchen knife, the glass shiv, the filet knife, the folding knife, or the Agency tanto knife.

18 - Berserker : Kill 5 enemies with an axe. It doesn't matter if you're spotted or not. This works with Axes, Fire Axes and Battle Axes. You can find them in a lot of levels, but the easiest to get is probably "Rosewood : The Orphanage" or "Shaving Lenny : Barbershop".

19 - Tank : Receive 20 hits without dying. It's highly recommended to use medicine cabinets to heal yourself (it doesn't reset the counter). You'll probably unlock this Style in the first rampage you get yourself into. If you survive long enough, it will unlock by itself!

20 - Jinx : Kill 2 enemies with accidents. This can be achieved in any level where there's more than one target, such as "Attack of the Saints : The Cornfield" or "Shaving Lenny : Hope Streets"
This can't be achieved on the level "Hunter and Hunted : Chinese New Year".

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I hope this will be useful ! If you have any question/trouble, do not hesitate to leave a comment. And if you want to add information/tips or correct me, feel free to do so!