I've got a fully upgraded thorn and in IB I've been able to 2 shot kill guardians with both landing headshot.

Has anyone else been able to do this? I use send it on the gun also.

And no, the poison didn't kill them, it was the second shot.

depends on their level and how they are speced out (armor vs agility for example). I've one shot plenty of lower levels with a sniper body shot without final round.