Hello Wary,
Fluency is the noun while fluently is the adverb.
Fluency: the quality of being facile and skillful in speech and writing of a language.
"In spite of being Chinese ,his Fluency in Japanese is remarkable."
fluently : in a fluent manner ,expressing oneself clearly, effectively and easily in speech .
"He speaks English fluently." ( in a clear, easy and effective manner)
diction:
The articulation of speech ,it is almost a synonym of "enunciation" in this sense.
It has another distinctive meaning which refers to the manner something is expressed in words; the way of speaking.
It could be both a way of speaking or writing.
enunciation :
is the articulation or pronunciation of words in speech, usually clearly and distinctly.
It is the aspect of pronunciation that involves bringing articulatory organs together so as to shape the sounds of speech.