If you are going to study independently, my advice is to be aware of the areas of English which give you the most problems: focus on your weaknesses, and then attack them, and destroy them! For each problem area, learn and understand the rules, then find practice tests on the internet. Practice, practice, practice! Also, for vocabulary, don't only study word lists. Practice with a lot of phrasal verbs as well. Native speakers use them all the time. (This is just one of many possible techniques, but in my experience as a student of languages and a teacher of English, it works very well.)

I've been learning English since junior high school, so I think taking English courses is the best way. Memorising vocabulary, grammar and taking loads of tests can push you to get there. There might not be a shortcut but you'll make it eventually.

A university course is the best solution. Plenty of hard work, time spent preparing exercises, and frequent testing. It is neither easy, nor fun, nor fast. If you take the "low road," be prepared for an even longer journey.