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The diet wall that everyone runs into.
Literal
Anyone-at-all [subject-が] crash-into diet [possessive-の] wall.
A 体言止め (noun-ending) construction giving this a magazine-headline feel — the final 壁 is a bare noun where English would need a copula. 誰しも is an emphatic form of 誰も 'anyone at all, everyone without exception.' The intensifying prefix ぶち~ on 当たる ('hit/run into') is a casual, punchy strengthener (cf. ぶち壊す, ぶっ飛ばす). 壁 'wall' is a stock metaphor for a plateau or obstacle in self-improvement contexts — the 'diet wall' is the weight-loss plateau everyone hits.