How can i get better?

[EduardoGaray]

Those X expression challenges are only useful when you doing a comic, an animation or something similar and you need to keep a consistent art style.
They can also be a cool way to practice many different expressions, but they wont teach you anything. The thing you need to do is to hammer the fundamentals.
Put a clear objetive, "i want to be better at this ..." Then break down the steps you need to take in order to achieve that goal.
For example, expressions:

-Draw a skull in every angle possible first from ref then from imagination.
-Once you have clear idea how a human skull looks and works, add muscles, study how facial muscles react to each other when smiling, crying, etc.
-Then add skin, repeat.

The best way to start is to get a mirror and practice from life using yourself, or if you find someone willing to pose for you, even better. :P
Also from photos, videos, anatomy books. But i would stay away from comics/mangas, you first need to master reality in order to stylize it.

Of course this is only an example, if you find this approach too tedious, you many need to find other ways to study. My point is, any skill is about understanding and repetition, and any problem can be solved if you break it into small pieces.

Keep practicing. :)