How does someone frag me with one shot?

[vonunov]

yeah, i know the feeling.
the best way not to get killed quickly is to move around ALOT!
try straifing in the most daft way possible. you will pick it up.

Pretty much this. Sometimes if you can tell your opponent is experienced, it helps to act more simply, like strafing just side to side or stopping dead for a moment to throw them off. Change direction a lot.