Jon strains to ascend the rope and join the fray. When he reaches the edge of the bridge, he draws his revolver and rests his arm against the bridge, slowly releasing his breath as he squeezes.

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focused aim
attack (1d20+8)[9] vs touch
damage (1d8+4)[9]

if I have the actions, otherwise attacking next round

assuming he has partial cover from the spider but free shooting against it.