To those who want it early, i got on chapter 3, just keep advancing destroying the robots and crates to mantain chains, when you get to the crate with the machine gun, keep transporting it,and only use on the big robots, i got 78 chains using this method and since i destroyed everything in the end got a lot of moneybecause a lot of aliens showed up, so you can kill two birds with one stone! use Paul Phoenix, no upgrades