Weapons and machine-guns themselves can't why the sisters are losing - it's more about figuring out how Reflect works. We saw that the Sisters hypothesised about Reflect not working from below because Accel could walk normally. I'm not sure how far they'd get (especially now that 10K have been killed with little difficulty), but once they do find a weakness, Accel might have to actually work.
The dude from Index-2 managed to thump Accel pretty hard since he knew how Reflect and vector calcs worked. In the end though, Accel's safe as long as he stays at a distance, turns on Reflect for protection and just throws shit at the opponent.