Well, I'd have several forms, in this order.
1st form: Just me. Not really all that tough, I look like a pudgy smart guy who leads from behind the scenes. You stab me once and game over, right?
2nd form: WRONG. Turns out I'm actually Dr. Wily in disguise, because it's always Wily. I pull out my latest robot and go to happy town all over your suddenly bleeding body. But, after Zero sacrifices himself, you take me down. But...
3rd form: I'm not out quite yet. I turn into an athletic version of myself, but wearing a suit of powered armor to give me all your abilities, but improved. I spend twenty minutes beating the hell out of you, telling you about how I'd guided your path along every step of your adventure, up to your inevitable failure. But then, before I can kill you off, your other friends join in and dismantle my armor with the power of friendship. Curses! Luckily, I keep going...
4th form: By cloning myself. Now one me fights each of your party members, exploring the depths of their personalities and their inner conflicts. One by one they prove victorious, and add their power to the final blast, which you use to eviscerate me once and for all.
5th form: But not quite. Now I'm a giant version of myself who rises up above the platform and starts using the environment to start beating the lot of you to a pulp; but in my arrogance I accidentally let you grow to my size and get beaten down myself. You've finally defeated me.
6th form: Except none of those were actually me. The real me is a tranquil, thirty year old or so man who is extremely man-pretty for no specific reason. My powers surpass your entire team combined. I beat down all your allies, but then get caught in a combo team-up attack that takes me down to nearly no energy left.
7th form: Good thing I set up that trap to let me fight you one on one. I beat the hell out of you this time, going for a brutally realistic show of what would happen when someone of my caliber fights a seemingly normal young man. I tell you you're all alone, but you respond that your friends will always be there in your heart and on your back, and out of nowhere just shatter my arm. I fall to the ground, finally dead, and you are reunited with your friends. It's time to celebrate!
8th form: NOT! Turns out, I was a pawn all along. Merely a worthless servant enslaved by the Reapers. They assume control of my corpse, shedding off its unneeded flesh and attacking with a new ferocity. One by one, your precious party is killed, from the funny young lad to the dark chick you brought out of her shell, to the wise and aged black mentor, even your second-in-command, there from the start, falls to my mechanical might. Then your love, the girl you've pined for since you were young, who has finally reciprocated your feelings in her final moments, is impaled on my spiked arm. You flip, you go fucking gonzo on my ass. I'm beaten down to nothing, and you make a beautiful speech on the value of human life, and how it will always be superior to machinery and coldness of heart. I am absolutely touched; even the mechanical shell that was once me feels for you, and in its last moments gives all of its power to bring back your friends, as long as the city I had destroyed prior to our climactic showdown. Congratulations, you've finally defeated me. But good luck with all those Reapers I work for...
