I am far from being an expert on this subject, but I also encountered the same difficulty, namely that certain words are not found in the Lidell & Scott dictionary. What I usually do is look into an English translation. This problem may be due to the fact that language had also evolved by the 4th century, but I would be curious to know more about it.
Don't forget also that different authors have their own style, but all were very much influenced by Attic. Plotinus' style is not that of Basil, and both authors also have differences with, say, Plato or Aristotle. Again, Gregory of Nyssa incorporates a lot of Classical imagery and poetry in his writings to an extent greater than any other author. I think that the style each author chooses depends on his purpose, on audience, etc. St. John Damascene, for instance, in his treatise "In defense of Images", uses a relatively simple style, which makes the work easily understandable by a larger number of people. Yet, Damascene's language is indeed Classical.