If you don't get this trophy in your first play-trough then start a New Game + on Narrative difficulty and play Priority Ops missions until you reach mission A Trail of Hope II (on Voeld in Nol system) section Hold out for the shuttle extraction where you have to hold out the enemy for 1:29. After the timer hits zero, don't go to the shuttle for extraction and keep on killing the endless waves of kett.
If you die just reload auto save and keep on killing. Yes the kill counter in statistics seems to be reloaded back as it was before this section but I got the trophy after reloading auto save 2 times so this worked for me.
P.S. Fiend has a chance to instantly kill you so don't get too close.