Perfect Shooter that anyone should buy!!!
There are many enemies Thuggish Splicers, Houdini Splicers, Spider Splicers and many more, but the main emery you need are Little Sister to collect their 'Adam' (which helps you grow new skills), the only way to get to the little sister is to kill their guard Big Daddies (which come in many forms Bouncers, Elite Bouncers and Rosies) they will do anything to protect their Little Sister. When you kill the Big Daddy you have the chose to harvest or rescue, bu whatever way you get 'Adam' I would rescue because when you have completed the game you get 100g for rescuing the Little Sisters.
You get Plasmids and Tonics. Plasmids are skills which re-write your genetic code, you can have skills like fire, electro shock, hypnosis, telekinesis and freeze, but there are plenty more. Tonics are skills which enhance your body skills, faster movement, stronger attacks, quicker hacking, shorter alarms and many more.
This Game is one of the best shooter games available; I recommend starting on medium if you are skilled shooter.