Screw Humans!  Robots are the best characters when it comes to strength and defense.  They don't gain status like Humans and Mutants, you have to "install" their status.  Various status ailments slightly increase each time you equip them with any item.  For example, weapons increase strength and armor increases defense.  Also, you can equip as many as you want of the same type of item.  Its very easy to make Robots so high in defense that attacks don't cause them damage.  So like, since Gloves are the cheapest form of armor, you can equip like 5 of the same cheap Glove and already you'll have rock solid defense.  Unfortunately, Robots do have their weaknesses.  They're totally weak against magic and special attacks, such as Dissolve.  Also, Cure spells and curative items have no effect on Robots, but I do know a little trick...

*SuperTip* When a Robot has low HP, equip and unequip them with any armor over and over.  Their HP increases each time you do that.  The stronger the armors you're using, the faster their HP will reach maximum.