The Siren's action skill is called "Phasewalk" and at first may seem like a weaker action skill than the other three, but may actually be one of the best. While active you are invincible, invisible and your movement speed is greatly increased. The damage dealing capability is limited compared to the other action skills, giving way to a more mobile and strategic gameplay. You get an area of effect blast around you when entering and exiting Phasewalk, and you can perform a melee attack that will force you to exit Phasewalk early. You cannot fire your gun, throw grenades, or do basically anything else.

To get the trophy you should find a weak enemy, like the skags near fyrestone. Use Phasewalk while near the enemy, then melee them. This should catch them in both blasts, which combined with the melee damage you should easily kill one enemy. By investing in the right skills you can do stuff like add bonus melee damage, or apply elemental effects to help you kill enemies, but you should not need to do so for this trophy.