Found this very useful guide, thought i'd share it!
Focus Fire Take down 10 enemies in a row finishing off each of them before doing damage to anybody else.
The easiest way to pull this off is to play as Nite Owl, and let Rorschach do all of the dirty work. While Rorschach is impaling upon everyone try and attract one to you, and "finish him off." The reason you play as Nite Owl is because you can block and that does not count as damage.
Unending Combo String a single uninterrupted combo that's at least 10 hits long.
During the course of the game you will unleash many combos, in order to keep a combo going don't do any knock down attacks and you will be able to keep the combo going.
Primal Scream Sustain a single rage mode for 60 seconds.
This is easier than it sounds. In order to activate Rage Mode you need to get the rage bar all the way up until it flashes between red and yellow. Once it is flashing, push left trigger and you will activate rage mode. I was able to do this as Rorschach in Chapter 1 in the big room of the Strip Club. In order to keep rage mode active you need to keep pulling off finishing blows to enemies, a finishing blow is when a button appears over an enemies head, and if you press it before he gets knocked out you will start a finishing blow. During this time your rage meter will freeze and will not go down. Once the finishing move is complete your rage bar will be raised back up a bit. Also throughout the game you will have the opportunity to collect Rorschach cards which will increase your rage meter.
You Tag 'em - I Bag 'em Eliminate an enemy that your partner has knocked down or thrown in your direction.
This can be done either in single player or in Co-Op, obviously it is going to be a lot easier in Co-Op since very rarely does the computer knock down an enemy without killing him. To do this in Co-Op just have a friend (or do it yourself) grab an enemy and knock him onto the floor and go and "finish him" while he is on the floor.
Street Justice Complete the game as Rorschach.
There are only 3 Chapters, and each one takes roughfly an hour to and hour and a half to complete.
Martial Artist Perform all 8 Nite Owl combos within 60 seconds.
This trophy is very difficult to pull off, there are a lot of things that can go wrong... And will, a finish move can pop up right in the middle of a combo with the exact button that you need to press, or you didn't pull them off fast enough, or you just ran out of people to beat the snot out of.There are 8 total combos that you must pull off within 60 seconds, this is best done in an area where there are a lot of enemies. In order to pull off a combos you must first initiate it by holding down the first button of the combo. EX: If you are doing the Knock down you must hold it down again to begin. You will know you have started the combo when you see them on the bottom of the screen. The first real spot in which you can attempt to get this trophy is in Chapter 1, right after you enter the big room of the strip club.
Ping Pong Hit at least 4 different enemies while building up a combo.
This is an easy trophy and can be done on chapter 1. The first part in which I was able to pull it off was right after you enter the big part of the strip club. Just let a bunch of guys crowd around you and start to pull off one of the bigger combos like: Extra Damage or Maximum Damage. Once you pull off one part of the combo just move the joy stick to another guy. In order to pull off a combo you must first hold down the first button of the combo, you will know you did it right when you see a blue or yellow circle on the bottom of the screen.