。
She arrived last.
Literal
She [topic-は] last [adverbial-に] came.
最後にきた is open to two readings depending on what 最後に modifies: 'came at the end (last in arrival order)' or 'came in the end (eventually).' Without context, the time-adverb-of-order reading — 'last to arrive' — is the most natural for a setting like a party or a meeting. 来た is written here in kana (きた) rather than kanji (来た), which gives it a slightly softer, more spoken feel. A pared-down sentence whose ambiguity is itself the lesson.