This is unlocked by beating any of the first 3 campaigns.
Aim for a Gasser's head and keep shooting until it kneels. Shoot again to finish it off. Kill them quickly because they can turn humans into zombies.
Play the final chapter of Chris' storyline and progress to checkpoint 5-1. Continue until you enter a circular room surrounded by levers. You will be ambushed, and each lever will spawn two Chrysalid B.O.W.s. You can pull the levers yourself. After killing all ten of them reload from the last checkpoint and repeat the process. Do not enter the next area or you will get a new checkpoint and have to repeat the entire first section of the level. Your kill progress will be saved even if you do not reach a new checkpoint.
In Jake's campaign, when fighting the helicopter in Chapter 4 at the part where you see Chris, get on the top of the roof. You will find some ladders and boxes to destroy. The helicopter will sometimes drop enemies on ropes. Get near one of those ropes and press [Action]. Then, climb up and shoot the pilot.
In Chapter 2 of Leon's campaign, after entering the cathedral, you must solve some puzzles to open the main chamber door in the center of the altar. Afterwards, you will encounter the Lepotitsa. It infects many humans in a short time by releasing C-Virus gas. You must save at least two females in the cathedral. To do this,, shoot at it as fast as possible while remaining at a safe distance. However, do not position yourself too far away or you will lose sight of it and it will infect too many people. If you do enough damage, it will expose its weak areas and get on its knees. When this happens, get close and perform a melee attack for increased damage. Note: This will require a large amount of ammunition under the higher difficulty settings. Consider doing this after completing the campaign, in order to have better weapons and perhaps unlimited ammunition.
Complete the indicated task to unlock the corresponding costume.Play with a friend or play with strangers but one thing is for sure, you HAVE to win.