The enemy you need to stab is the "Licker". The earliest opportunity to do this is chapter 5-1.
There are two ways to do it:
If a licker jumps on you and pins you down, and your partner doesn't save you, you die. However if just before the jump you take advantage of the prompt to uppercut it will knock the Licker onto it's back, at which point you can stab it and you will instantlly get the trophy.
Otherwise, use your shotgun on the Licker and flip it on its back, now run up to it and use the stab prompt.