I'm not that great at PK and I don't usually have a hard time killing assassins as a warrior - from my most recent warrior PBF (do note that 2/4 assassin deaths were assassinates..possibly 3/4)
PK Wins by Class
VS. transmuter 3
VS. warrior 41
VS. paladin 5
VS. anti-paladin 1
VS. assassin 12
VS. ranger 1
VS. invoker 5
VS. necromancer 3
VS. shaman 6
VS. conjurer 1
VS. shapeshifter 5
PK Deaths by Class
VS. transmuter 3
VS. warrior 5
VS. paladin 5
VS. anti-paladin 3
VS. assassin 4
VS. ranger 2
VS. invoker 2
VS. necromancer 3
VS. shaman 2
VS. shapeshifter 2