Duolingo works great as an introduction to the language, but you can't really expect it to bring you to fluency, no matter how much time you spend strengthening skills or doing the reverse tree.
I like to view the process of learning a language in terms of competencies. If you've only finished a Duolingo tree, you will most likely need a lot of additional practice in speaking, listening, and reading long-form texts to bring them in line with your current vocabulary and grammar skills.
The most effective way to improve your speaking skills would be simply talking with native speakers of Spanish, or even other learners. I recommend checking out HelloTalk, Speaky, Interpals, or Meetup (for in-person meetings) — it shouldn't be too difficult to find someone who would like to chat with you in Spanish.
When it comes to listening, you need to immerse yourself in Spanish-language content. TV series, movies, podcasts, radio, audiobooks — whatever works for you. Try to start with something that comes with subtitles or transcripts and listen attentively. When you begin to feel more comfortable with spoken Spanish, you can add passive listening to the mix — try listening to podcasts or radio in the background as you do some mindless tasks.
The trick to improving your reading skills at a basic/intermediate level is choosing the right genres to practice with. I suggest starting with things like children's books, comics, books adapted for language learners and introduction sections in Wikipedia articles. All these tend to be written in a relatively simple language and should help you ease yourself into reading more challenging text types.
If you'd like to see a more comprehensive breakdown of dozens of learning techniques that should work well for anyone who's finished a Duolingo course, take a look at my blog post What to Do After Duolingo: The Definitive Guide.