When you couple external traffic with an effective Amazon PPC strategy you'll find keyword ranking to be a much easier task. Though the strategies should be used separately if you want verified reviews, together they are a potent combination of diversified sales traffic.
I see a lot of sellers spending a fortune using Amazon PPC without any real progress in keyword ranking, which Sponsored Ads should be leveraged for. You want to rely on organic sales from strong keyword positioning rather than Sponsored Ads alone. If you're too dependent on Amazon PPC for sales you'll quickly find your cost's going up without any organic traction to support those costs.
I was recently on a webinar with Augustas from OrangeKlik going over top strategies for Amazon keyword ranking and Amazon PPC Optimization. On this webinar I went over in great detail, step by step how you can optimize your PPC campaigns for both profitability and page one results.
If you want to speed up the process of ranking for keywords I encourage you to look into our Amazon Launch Service, which is in align with Amazon's TOS and our Amazon PPC Management Service where we'll help you quickly scale your revenue with consistency, month after month!