Challenge Mode is no easy task. I ve managed to beat score three times higher than gold requirement. Didn't need upgrades because this strategy works fine. First, round up some kills by blowing barrel, setting traps then run for the roof. It is hard to cross gap on wood without falling down so it takes patience. When on roof position on right side and switch between heavy gun and throwables for large chain. Chain is most important thing for gold, I ve managed to get more than 400x multiplier and literally smashed the gold requirement. Image is little different due to HDR settings.