How to dissolve bronze at room temperature
Supposing that the bronze is only 78% copper and 12% tin (and there are no other metals). Nitric acid will dissolve the copper... but this reaction is exothermic, which means it will heat up. 

At boiling temperature. Nitric acid will also react with tin to make tin oxide which is soluble. However, if you kept it cool/room temperature with an ice bath, then you would have leftover tin which did not react.

Tin will react with aqua regia to form tin (IV) chloride, but copper is slower to react with aqua regia... so it would be better to use nitric acid first (to dissolve the copper) and then add hydrochloric acid to react with and dissolve the tin. Once again, the reaction between tin and hydrochloric acid (or aqua regia) will proceed much faster with heat, but it will happen at room temperature.

Aqua regia = 1 mole of Nitric acid and 3 moles of hydrochloric acid).