Well, to answer your question, seeing you mentioned Nature runes, I'm going to assume you're a member. In that case, buy a whole lot of dueling rings and do fires. Reasonably fast exp. But, I don't get what you mean by you can't afford enough pure essence for 91. Buy it in loads. Buy maybe, 2k, 5k, 10k, whatever you can afford, and think you can do. Then sell the nature runes and buy some more pure essence. Even if you're not using the abyss, it will be faster (I think), and definitely make you more money. Just make sure you don't do more than you can handle. To be honest, if I were you, I'd go for a smaller goal, like 75, first. Then gradually increase your goal. You have more chance of reaching it that way.