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Let's say you happened to hit a long-shot win at the races.
Literal
Supposing, horse-race [at-で] by-chance long-shot [object-を] hit [hypothetical-としましょう].
Setting up a thought experiment. 仮に ('hypothetically,' 'let's suppose') is a formal/written hedge for marking a statement as a hypothetical scenario — often paired with ~としましょう or ~たら. ~としましょう is the volitional of ~とする ('suppose that X') — 'let's suppose X,' 'let's say X.' A more polite, cooperative version of the bare ~とする. たまたま ('by chance,' 'coincidentally,' 'happened to') marks the event as unplanned. 大穴 ('long-shot win,' 'big hit,' 'upset') is horse-racing vocabulary for a big payoff on an unlikely bet — 穴 ('hole') here figuratively for an unexpected gap or opportunity.