As the description says for this Trophy you will need to get a Streak of 8 Guardian Kills/Assists without dying. Focusing on the Assists will take the frustration out of this Trophy. To do so I would recommend selecting Guardians with damaging AOEs or healing and buff abilities (healing an ally while they kill an enemy Guardian counts towards an Assist). Probably the best two Guardians for this are Hildifons with his 3 different AOE attacks and Galadriel for her healing/buffing. Galadriel might be your best bet since she has considerably more survivability than Hildifons. Just hang back, don't take chances and this Trophy will come.
As a note, I got this Trophy with Sauron, His Ultimate damages all enemy Guardians. Just keep any eye out for low health Guardians and pop the Ultimate. Worked for me!