If you don't complete this during your normal playthrough then after you have finished the game go to chapter select, choose chapter 17 and then enable infinite ammo, one shot kills and you weapon of choice.
Once Elena lowers the rope just stay down at the bottom of the pit at the enemies will infinitely spawn, usually three at a time and just keep killing them until the trophy pops.
I did it on easy and you kill need to watch yourself as two hits in quick succession can kill you.