That is really cool! What is the new solution? As far as my story goes, I have to make her bleed out all of her immortal blood or keep her dosage of it away long enough to make her mortal again, but she is incredibly strong in battle and is far too smart to lose her concoctions so easily.
Throw them in a lava pit continuously until i get bored and then force them to watch The Green Lantern film with Ryan Reynolds in it fifteen times.
I'm partial to the separation of his body, if that's possible (Not sure how 'immortal' the character is), scattered to the corners of wherever he's been doing naughty things (A la Braveheart), but assuming his head would still be animated, or that you could even sever it, a talking head on a pike could be an interesting twist.
Thanks My new solution? I don't have one. He's still alive. He became the co-MC... LOL. Yeah, he and my MC befriended each other in the end. In fact, my kickass immortal "villain" came and saved my MC's life. hahaha
I'd turn them into a goldfish. You know, because what's a goldfish gonna do? Open and close it's mouth in rapid succession at you?
It is simple. EXTERMINATE EXTERMINATE EXTERMINATE! Kidding of course. You could always chop off his arms and legs (Come back here and take what's coming to you! I'll bite your legs off!) Lock him into a time loop, throw him into a volcano, fire him (out of a cannon into the sun), chuck him into a black hole, turn him into a roadrunner and stick him in a cartoon with a coyote for all eternity, there's tons of ways to get rid of an immortal.
First, I would unfriend him on Facebook. And if that is not enough of a blow (for most immortals that I know, is reason enough to attempt suicide, as they know the one thing that can kill them) I would do what I can do to trick them to reveal their vulnerability, and or ask another immortal to kill for me and or get myself on the witness protection list, change my name etc so that I am invisible (for a while) until I come up with a better plan. My theory is though, that all in this universe is created in such way that any negative has a positive so things balance themselves out. Even immortals die. If they venture to another astro-physical plan of existence, or if the mortal does, is same. But, Dr.Who would know better.
Why does the kill have to be physical? Why lock in a cage or underground when he has the millennium to await his eventual escape? There are many clever ways to dismember an immortal, or just deus-ex-machinia it and discover some weakness. Wouldn't it be more interesting if the villain was convinced to stop being who he is? If this immortal has been fighting something or for something or towards some goal, then what happens when the very reason he is a villain is stripped away? What if the MC can convince the immortal that his actions and goals are not worth it? What is time to an immortal? What could be so important that someone who has eternity to get what they want has to press to get it now? What drives an immortal to do things when he isn't pushed onward by the countdown above his head signaling his eventual demise? Take everything the immortal is fighting for, and take it away; and you have an eternal being who will live forever no matter what he does who for once has no reason to live. For once all his actions, past and future, seem worthless. Or for once he sees the error of his ways, and walks away from the situation and into the whispers of myth.