If you're F2P I suggest you train against Al-Kharid warriors, Monks, Hill giants and/or barbarians.
If you're P2P you should use bronze knifes to train against Fire Giants ( high HP levels and good drops), Rock Crabs (popular place to train due to their HP and low strength and defense), and/or Yaks (mixture of both reasons stated above).
EDIT: If you have time, you can also use mini-games like Pest Control or Soul Wars to get points in exchange for experience.