Duolingo is a feel good tool - it makes you feel like your learning, perhaps quickly, without really learning anything.
For example, it uses multiple choice questions. Presented with a word (ananas), it then shows a few pictures (apples, bananas, pineapples and pears, for example). You incorrectly choose 'bananas', because really who wouldn't, and the pineapple lights up in green.
This method is really at odds with the 'best' ways to learn. If your recall is dependent on contextual cues, you will struggle away from that context.
Additionally, the volume really isn't that great. Like a lot of 'feel-good learning' platforms, duolingo encourages consistancy over volume. It's better to study for 10 minutes a day, than for 90 minutes every sunday. That's true, but if you want to learn a language (no small feat), then even 90 minutes a week is woefully inadequate. Being conversant in multiple languages is something to be proud of, and things to be proud of tend to take substantial effort.
Vocabulary acquisition is almost the perfect problem for spaced repetition, so use anki. There's a few strategies here: english word -> foreign word, picture -> foreign word, foreign word -> foreign synonym, foreign word -> english options, english sentence -> foreign sentence.
Pronunciation is difficult at first but gets easier. Accents are tricky - they're verbal gymnastics. Its easiest if you can find a native speaker of your target language, and you should just converse with them. Don't focus on minor details, because you usually pick these up naturally over time.
Listening is easy - listen to their radio, watch their TV. Similar with reading.
But there's one thing I haven't mentioned. Grammar. You do not need to learn foreign language grammar. You'll pick it up. How often have you studied English grammar? Unless you went out of your way, probably never. You learned some basic rules in your first few years of school, but everything else you picked up.