Well, how about one that's virtually unkillable... a mini jetpack spidertron-6000 wielding a hammer :3
How about the Emperor of Robots himself? The humans would get to see his shiny neon back hip plate!
(Human) oh, god no...
(Analysis-Bot) *oh, robot no...
(Emperor) *shows the neon*
On a side note, I'm guessing that far, far in the future the Emperor might be some kind of boss in story mode :3
It should be the final boss, and be REALLY tough to defeat
although, he IS very large... oh, just create a smaller clone of him and transfer his mind to that clone
or, you know, just let the human figure things out for itself
Maybe the human would have to climb him, and hit a core on his head!
...or hit the shiny neon back hip plate xD
