First, lets be honest, a guard cannot one shot you. Unless you are level 35 and he is 80 perhaps?
Second, how does a guard having a healer improve this one shot you are talking about?
A guard should not be your primary focus. And probably why you are confused. Any good player in pvp will tell you the guard should be CCed and ignored, while you kill their healers and casters. If you are focusing guards in PVP, then there is a whole other thread we should start on primary targets, SW necros, and the real threat of a healer.