@nondirectbacon
You do not need to use any 'complex' words or try to spice up your vocabulary. If anything, if you use complicated vocabulary you may end up confusing yourself.
Here is the feedback I got after my ICAO test (I got a 6)
Comprehension
(6) Displays accurate understanding in a wide range of contexts
Fluency
(6) Speech flow is confident and natural at all times
Interactions
(6) Interacts in a relaxed way
Pronunciation
(6) Separation of sounds is appropriate
Structure
(6) Structure is fluent and adaptable
Vocabulary
(6) Explanations are concise
Hope this helps
