For this achievement, you must complete the game on co-op. Below are some notes and tips for helping with unlocking the achievement.
-At time of solution, this game has local co-op only.
-This achievement will unlock in conjunction with the achievement for completing the game on easy/normal/hard difficulty. (whichever you choose when starting your game)
-Co-op mode must be initiated at the start of your game. There is no way of starting a single player game and having a 2nd player jump in.
-You will not unlock the Mike or Dorian achievement while playing this mode (at least, I was using Doran as player 1, and did not get the achievement upon completion).
-I was able to beat this solo, using two controllers on medium difficulty. It is certainly doable, but some stretches are pretty tough. I would recommend playing through the game on single player first (easy or normal), and then attempt this afterwards (alternating to the other difficulty).
-You can get the "Fists of Fury" achievement in co-op, but I would strongly discourage even attempting it if you are playing on Normal or Hard. Using the gun is very helpful in this mode, especially to weaken or stun large groups of enemies. If using two controllers solo, be sure to use player 2's weapon as well when needed.
-Whenever you revive your partner, they will get 1/2 of their life back. (Although they will lose some of their score). In situations in which there is only 1 enemy left on screen and your partner has less than 1/2 life, you can have the second player get themselves killed. Now you can kill that enemy and revive your partner to manufacture a little extra health (you need to be careful though, sometimes more waves of enemies appear. Plan accordingly)
-There isn't generally enough score per level for both players to earn RP. I highly suggest letting one player do most (if not all) of the killing and break all of the token boxes. You can then replay the stage and switch out so the other player can get earn their RP.
-The second player (especially if you are playing solo with two controllers) makes a good decoy. If you can get some of the larger enemies (dogs, cows, etc) to focus on the second player, you can often string long, uninterrupted combos on them.
-Boss fights can be done solo (let your partner die). I recommend this anyways, as two players make for odd camera angles and can sometimes make the boss patterns a little unpredictable. Bosses in this game use somewhat simple patterns that, once learned, will allow you beat them with little trouble. Make sure to utilize your guns, conserve med kits (use the revive tactic if the boss fight allows), and take your time.
-Focus on upgrading your gun to max. I finished the game with max gun, and one slot missing for combos and shield.
-Practice, practice, practice. I highly suggest having already played though on solo to get a good idea the different enemies and good strategies to beat them. Get efficient at reflecting bullets, countering punches, and rolling out of the way when necessary.
-Spam the stomp combo (Y,X,Y) on some harder enemies. You can often times knock them down or stun them with this combo and then repeat it over and over until they are dead. Also has decent area damage and is quite useful if you find yourself completely surrounded.
I hope this information helps. I will be more than happy to add additional stuff if anyone has recommendations. If you vote negatively, please leave feedback. Thanks, and good luck!