Feeding Frenzy Trophy

POSSIBLY GLITCHED: Although it sounds like this should pop once you've used all 14 once, it seems that this is either not the case and is instead for reaching a certain number of fatalities used or the trophy is glitched. I did eventually get it but only after dozens of times using each fatality so after you're certain you've used each once, if the trophy still hasn't popped for you, just keep killing using fatalities and keep cycling through them until you get the trophy. 

There are a total of 14 Feeding Fatalities in the game and you'll have to use all of them at least once. 10 are available to you right away as you begin the game and the remaining 4 unlock very soon during the tutorial in the beginning, once you get your guns (marked with an *). To Feed, you have to approach an enemy within melee-distance and press . While feeding, you can then press another button and it will initiate the fatality. Basically as it sounds, a fatality will stop the feeding at that moment and kill the enemy in one hit. 

Fatalities using  and  will give you Blood Rage, while fatalities using  will give you ammunition for your guns. The ammount you get for either is directly related to the amount of health the enemy has left at the moment you activate the fatality. Note: The ++ Fatality activated from the enemy's back gives the most Blood Rage of any fatality.

TIP: There are a few great spots early on with infinite (or nearly) enemies to do them en masse if you want an easy way of making sure you've done each of them. One of them is Act 3: Union Station - Club Strages where you are stuck in a bloodfield and have to harpoon enemies into the stereo speakers. Until you complete the puzzle, enemies will continue to spawn infinitely so this is a great place to work through each fatality. 

The 14 fatalities are split up in two categories: From the front and from behind, i.e. some fatalities can only be done when approaching an enemy from behind and jumping on their back. Here is a list of all 14 Feeding Fatalities:

FROM THE FRONT:
 : Devil's Handshake
 : Sterling Blow
  : Laughing Jackal
 : Archer's Bliss 
  : Cracked Sparrow
 : Silver Eeel *
  : Athena Shiver *

FROM BEHIND:
 : Strangled Flame
 : Lament Reversal
  : Crane's Reply - Note: Gives the most Blood Rage of any fatality.
 : Rabbit's Lash
  : Comet's Twinge
 : Weeping Cobra *
  : Lover's Grip *

Note: Fatalities marked with an asterisk (*) will be unlocked once Rayne acquires the Carpathian Dragon Pistols in Act 1, Zerenski's Mansion.