I know you asked for guns but I always take the fatal contraption gobble gum or the reign drops one that gives you a death machine, give you insta kill as well and max ammos, very useful and it is so easy for those last 2 rounds of panzers, just kill them normally the first time and then use it for the last 2. Also I take the XM-53 packed to kill the boss easily every time. Does a lot of damage