Maybe you only have a few hours a week to prepare for HSK testing? That's actually enough. That is, provided that you know the fundamental sentences patterns and vocabulary that the HSK is testing. Learning vocabulary is the most simple strategy to prepare for HSK testing. Remember that for the listening section of the test, different photos are presented, and you're asked to match the heard sentences to the photos. Thus, a large vocabulary is a useful resource for anyone preparing for HSK testing. Your journey toward passing the HSK test is a road paved with vocabulary and special study skills.
If you want to learn mandarin Chinese, that's one thing. But if you want to pass the highest level of the HSK, that's another skill. Our students tend to get their level to passing the HSK at around 3 years. The HSK is funny like that. You can prepare for HSK on its own and even pass it without having a decent Chinese ability.
The number one strategy to passing the HSK is to memorize vocabulary. But isn't this also the number one strategy to learning Chinese? Kind of, but not really. Many teachers recommend flash cards to help students connect their native language to Chinese. But for the HSK and even for Chinese in general, we don't recommend randomily preparing flashcards. For the HSK test, we prefer to group words. For each phrase, you should memorize them with similar phrases and Chinese characters. In this way, you won't have to memorize loads of words that start with "A" or "Shi" at the same time, which is how most flash cards organize their vocabulary. That's why we recommend phrase books that are organized by context and situation as opposed to dictionaries, which are in alphabetical order. In this way, when you drill yourself, you'll be creating neural connections between similar words (not sounds!) in your brain, which makes it easier to create sentences and write sentences for the HSK speaking and listening. It helps in the listening section of the HSK, as well.
Once you say you are going to prepare for HSK testing, you are going to have to learn to write. The Chinese characters can be the biggest difficulty for many HSK test-takers. This is why you need to write as you memorize vocabulary. It's easy to sit in front of your vocab lists and practice reading them, but it takes more effort to get out a pen and paper and practice writing what you see. You will find that you almost never remember the characters you want to write when you need them on the HSK, unless you've practiced well in advance, that is. While proper teaching and good studying materials go far, you really need to practice writing as well. As you learn Mandarin Chinese, you have to actually apply it, even if you're by yourself. Once you get the hang of it, you'll find that there's not as much complexity in the characters as you originally thought. The essential factor is to practice daily for the sake of HSK testing.