My fantasy for this type of class usually leads to a dynamic where you are constantly upgrading it over the course of a battle. For example, a party heal skill that permanently increases the Robot's Max HP for the rest of the battle, an expensive attack skill that also makes the Robot's single-target skills into AoE for the rest of the battle, a passive trait that increases the Robot's ATK stat for the rest of the battle every time he kills an enemy, and so on. The robot can start battles off pretty weak, and by the time you've stacked several of these upgrades onto him, he becomes a juggernaut by the end of the fight.
This type of design works best when you, as the designer, have mechanics in place to generally control (and limit) the length of battles. It could be problematic if your normal encounters last 2 turns and your boss battles last 30 turns, unless you have some kind of mechanic in place to control the pace at which the robot accumulates upgrades (perhaps instead of lasting until the end of the battle, they last until the robot has taken X amount of total damage). But in a blue-sky, initial design phase, this kind of semi-permanent upgrade structure is what sticks out in my mind as a really cool way to create a robot/mech type of class.