My main I have now was originally a pure. I botted all of his combat stats to 50 at seagulls and minotaurs, and I did quests like Fight Arena and etc for XP. Then I just trained at Experiment's from like 50-70. I finally decided to become a main since my previous one had been banned(it has now been unbanned like 2 years later lol...)at like 70 strength. Experiments are pretty decent, like 30k+ xp/hr depending on how many people are there.
My opinion is why even bother using Soulwars to level.. why not just train on Zombie/in TzHaar Caves, quest, etc.
But if you're serious start at 75-80.