Immersion is most powerful. ie living in the country, forcing you to live, eat, breath the language.

However as described, a mixture of techniques is needed. Writing, reading, talking, listening. Plus practice, practice, practice.

There are a few examples of people learning languages quickly... Like get fluent in 3 months. Once you understand the mechanics it is easier.

Also some languages are easier than others. Germanic languages 'should' be easier for English speakers.