imo, the Qin army could win by maneuvering, having outnumber Alexander and having a lighter hence faster center (phalanx = powerful but slow center), they can do an envelope; and having superior archery, it would be hard to charge, and counter flanking would be slow against a long line.
the only way i know of, to beat a larger force with superior missile force is to shift your center toward the flank and concentrate on it which would be hard since it would break the phalanx and take time to reform the line.
the calvary charge is the most powerful attack Alexander has, and the only thing i feel can counter it would be poisonous arrows that can kill what would normally just wound you. so i like like to know if poison arrows has been developed and militarized during Qin.