When it comes to describing someone who is fiercely intelligent, there are plenty of synonyms to choose from. For starters, you might use terms like highly intelligent, brilliantly astute, or razor-sharp. Those who possess this kind of intellect might also be called intellectual powerhouses, which conveys both the strength and sophistication of their cognitive abilities. Other possible synonyms for fiercely intelligent might include quick-witted, shrewd, perceptive, or discerning, as these all capture different aspects of intelligence and mental acuity. Ultimately, the right synonym will depend on the person you're describing and the context in which you're using the term.