This matter can't be accomplished by one person's strength alone.
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Literary/formal. Almost always appears in fixed patterns with 之: 一己之力 (one's own strength), 一己之利 (personal gain), 一己之私 (personal selfishness). In everyday speech use 自己 (zìjǐ) instead.
grammar
The 之 here is a classical genitive particle equivalent to modern 的 - so 一己之力 = 一个人自己的力量 ('one person's own strength').
