Story based trophy, should pop after completing mission 4.
*** Spoiler - click to reveal ***You start by visit g Tannis and killing a few waves of COV. After there’s a driving session for Ellie where you need to run down enemies, you can stay in any area where skags respawn and farm them. Finally, another few waves of COV need shooting and then Lilith needs reviving.
Nothing too harsh, level 7 felt about right for the mission.