Get a basic assault rifle that you are for sure going to be lugging around awhile and a basic +5 shock damage weapon mod. Sometimes you need a lot of bullets to kill some of the robots. The earliest in the game you do this the better. I think that Edgewater's general store sell the mod. Put it on the gun and every time you come across a robot, just kill the robots with that gun after spraying them a little first so the shock damage takes effect. Not all of the kills count but they add bonus damage exp when they do. I killed about 40-45 robots with just that gun before it popped. Ludwig on Edgewater has some missions that are great for this in the beginning of the game to start this on. It requires a little patience but it works.