Plugin is a word that comes from plug in, like you plug in your charger and electricity flow to charge your cell phone. similarly plugin for website will only help to automate something in your website. There are some plugins for SEO as well and yoast is one of them, an old plugin. I am not sure hot good is it now a time but after the changes in G Algorithm there is no such plugin which will help you to rank in G. You have to learn SEO and have to implement it. As far as you said that your keywords are HARD, can you share your keywords and tell us how do you think your keywords are HARD ?