。
She's strong-willed.
Literal
She [topic-は] mind [subject-が] strong.
気が強い belongs to a tightly bundled set of 気 idioms describing personality: 気が強い ('strong-willed, tough'), 気が弱い ('timid, weak-willed'), 気が短い ('impatient'), 気が長い ('patient'), 気が小さい ('cowardly, small-minded'), 気が大きい ('big-hearted, generous'). The pattern is consistent — '気 が [adjective]' = '[adjective] in spirit/disposition.' For 気が強い specifically: it's not exactly 'aggressive'; more 'won't back down, holds her ground.' It can be praise or criticism depending on context.