Kill Stealth
Kill enemies while they are Cloaked
I See Cloaked People
Kill enemies while they are Cloaked
ESP
Kill enemies while they are Cloaked
Cloak Tracker
Kill enemies while they are Cloaked with Cloak Tracker equipped


Stealthed Assassination
Stealth kill enemies
Assassin
Stealth kill enemies

Then there are the 4 or so challenges that revolve around melee kills..

It's kind of easy to tell you why.... Crytek has no creativity?