Are you going to be F2P PK'ing? Or going to go P2P?
For F2P, just keep getting that strength up along with range and start range->2H'ing people for KO's.
If you go P2P, I would recommend questing your attack up to 50, and using G-Maul special attack for KO's. For this as well, I recommend ranged to use rune knives/Magic shortbow to lower your opponents HP and then special attack for the KO.
Also, you could do alot of questing, and go for addy gloves (RFD gloves 6 <- I think). This however takes quite a bit of dedication and time.
Its good you have lower HP, because you can now have higher strength and attack for your level. Questing will also have this affect (gets str/atk exp without getting HP exp).