Techniques for improving listening comprehension under background noise conditions
Try your best. 

Similar to any language that being "localised", it took "origin" speaker time to adapt in order to understand different slang. E.g. A GB English speaker will have a hard time following Australian English, New Jersey slang, Mississipi slang.

If you watch series from PRC, it is mostly Beijing slang. If you don't understand most of the words speaking by some Chinese, then you probably hearing some sort of dialect, e.g. Shang-hai-hua, Guang-dong-hua ,etc.