For a melee pure, Attack is key, trust me. Strength should be nearly maxed, but in my opinion keeping attack level close to strength is important. Combine that with 1 Defence and you have a deadly pure. Plain and simple. In my experience, having 1 Prayer is really nice up until a certain point. Ideally you want 31 Prayer for maximum damage output for your combat level. However, due to it being annoying to recharge and the fact that it doesnt really help until higher combat level, I would keep 1 Prayer until your around 95 Strength.
I would go for
40-80-90 Attack
90-95-99 Strength
1--31-31 Prayer
The damage output of this account will be really high because it is not only accurate but powerful with good KO potential. Put a gravite rapier in the hands of it and you probably will combo anyone to death if they are no arming you... Gravite 2H will shred through rune pure's armour.