My players had very little trouble with the creature. My gun wielding arcane gun/magus put a couple shocking bullets into it, while my warpriest of Brigh used his artifice blessing to ignore hardness and it was down in less than 2 rounds.
I think by level 3 the players should have more than enough resources to take out this robotic menace, but if not as it was said earlier, you can bypass the fight and shut down all of the robots later in the scenario and then go back.
Umm ... the shocking bullets wouldn't have helped.
Hardness counts as both Electricity Resistance and damage reduction.
The shocking part of the bullet only does a maximum of 9 points of damage. (1d6 x 1.5 for vulerability) and 9 - 10 = no damage gets through.
Now, a Musket could get through Hardness on a high roll (doing 1d12 damage), but even so, he would be blocked by the Hardness 5/6 of the time.
Also, I am not familiar with the Arcane Gun magus archetype. I do know the Spellslinger Wizard Archetype, is that what you mean?
Perhaps I ran it wrong but I figured the bullet and the electrical damage would be added together. Throw in some various bonuses like bard buff, point-blank, etc and decent rolls put his hit at 15. The more I know.