Every language can be spoken fast and it often sounds unintelligible to foreigners, but Sapanish is one of the hardest because is one of the fastest spoken languages only topped by Japanese.
In Spanish we have to pronounce almost twice as many syllables as in English. So my recommendation is that you search Spanish conversations where they speak fast, preferable fragments from movies or series you like, with Spanish subtitles so you can learn what it says.
Then you have to record the phrases that you don't understand, you may use your cellphone for this, and repeat it as many times as you need, until it makes sense. Eventually you'll be able to understand it at any speed.
Try to replay it along the day whenever you have free time, and get used to the common way of speaking the language.
That's what has helped me out to learn hard phrases. I hope, it helps you as well. Good luck!