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She was a scientist of unprecedented greatness.
Literal
She [topic-は] up-to-now precedent-not-existing [extent-ほど-of-の] great scientist was.
Two impressive grammar pieces. これまでに ('up until now') sets the comparison frame, and 例のないほどの ('to the extent of being unprecedented') turns 'no precedent' into a magnitude marker — 'great to a degree without parallel.' Notice how の attaches twice: first to nominalize 例のない into a modifier of ほど, then to attach the whole phrase to 偉大な. 例 here means 'precedent, instance' rather than just 'example'; 例のない is a stock pattern for 'unprecedented' alongside the synonymous 前例のない. 偉大な is a heavyweight na-adjective specifically for great achievements, the kind reserved for major historical figures.