If I were you, I would try to do many reading comprehension exercise to improve my reading, as well as my writing by investigating how the author writes the text, for instance, how does the writer use a wide variety of 論證手法 to make the text a persuasive text.
But I am weak in listening, I am still working hard to improve my listening skills. Thus, I can't tell you any skills to improve listening now.
For speaking, I advise you try to link your points when speaking, using connectives and other kinds of connecting methods, like "firstly", "secondly", "in conclusion", etc. On the contrary, I think we need not to use a 古語, provided that we have a clear stance, persuasive points and a clear organization. In addition, we should be confident when we are speaking or having face-to-face communications with others. I suggest you joining some public speaking classes or announcing messages in the morning assembly.
Work hard! If you work hard, you will be succeed!
These are all my learning experience which could be wrong for learning Chinese. Consider my suggestions as references only. I hope my suggestions do work for you!