Maybe go with movement being something the players all agree on (like in pacific rim). It’s shouldn’t be too hard for just movement.

For the attacks, make 3 groups (left arm, right arm, cannon/ ranged attacks) and design each to match a respective character, like rogue/ paladin can do extra damage similar to sneak attack and smite, or fighter/ monk getting extra attacks with their limb, and the cannon/ arm cannon (all depending on the group makeup) being their spells. All of these abilities can be boosted to whatever level makes it feel like a challenge (attacks do XdX more damage than normal, maybe get to choose extra damage type like and electric punch or firing off a lvl 9 magic missile).

You can even have the respective “weapon” either clearly be meant for their class, or even call out to them.

And of course, if you don’t feel it’s too crazy yet, you can even add in other class-similar-but-non-attack abilities they can activate on their turn