Small correction on enemies that are vulnerable to critical hits: Robots don't take extra damage from critical hits, they just have to make a DC 15 fortitude save or be stunned 1 round, or staggered on a success.
Since we are talking about module 6 at this point, enemies are going to be very effective and tactically sound, assuming they aren't mindless. Your barbarian is relying on a series of attacks to properly do his Barbarian level damage, so any capable enemy will ensure that he won't be able to full attack and will have to work to get each hit. If he doesn't have access to flight, the enemies can just fly away for example. The Overlord robot will run right over the barbarian if he just runs in, and the Godmind fight is all flying creatures that will gladly stay out of reach.
What is giving the barbarian such high saves compared to the rest of the party? Barbarian's will make the fortitude saves, that is generally a given, but even with the bonus from rage, the Will save will still be dragging a bit, and Reflex will be worse especially with the low dexterity of an Armored Hulk.