In all seriousness, he's kept himself alive through hatred and pain, so if you make him feel comfortable or happy (somehow), then you can get the job done.
Jumanji is a habitual quitter. He'll come in, say some wildly inaccurate shit about anything that could possibly interest Jumanji at that time, and then throw a little hissy fit and leave once nobody listens to him.
If he were in the Star Wars Universe he'd be a Youngling destined to turn to the darkside. Only he never achieved anything because Anakin just walked through the door.
All the stuff about eroding his willpower is pretty obvious. Sion keeps himself alive through the Dark Side, naturally. Its not the willpower on its own sustaining him in the sense that it would obviously not work if he wasn't Force Sensitive.
Anything restricting his connection to the Dark Side would halt its constant flow keeping his dead body alive. Anything disrupting the energy flow to a properly large scale would do the job.
Force Light would do it, if the effort was concentrated enough. Force Light is basically the pure Force energy destroying and disrupting the tainted Dark Side energy. With magnitude proper enough, Force Light would disrupt the flow enough to kill him. He needs constant connection to the Dark Side, as it is the thing that literally holds his body alive.
Wall of Light is just the same in mechanism, but of a higher magnitude and is basically a "full" Force Light. It kills him instantly.
Sever Force can do the same thing, so people like Vergere, if they are given enough time etc. can drop his connection enough for him to die.
Drain is a wildcard given how weird and complicated are all the mechanisms of the drain surrounding the Sith Triumvirate, so I won't go into detail here.
Then there is the obvious aspect. Force is energy, everyone has his upper limit. You see it in their upper limit in Force Speed, telekinesis, in healing. Force users, their connection to the Force, their minds, get used up and depleted. Force users can't sustain Force usage of a very high level infinitely, and after certain amount of its usage they can't hold it any longer, be it their mind losing concentration, their bodies unable to sustain the energy, or their potential being depleted. So obviously Sion can get killed by natural means if enough punishment is served in a specific amount of time. He can hold his body together via the Dark Side suffering 20 lightsaber thrusts to the limbs across 2 minutes, but it's another thing when you are getting ragdolled into ray shield 200 times in span of 30 seconds. But where Sion's limit is? It's obviously both unknown and immensely far.
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They are angry because they have been forced to recognize that their hour has arrived; that the time has come to surrender power to Shimrra and the new order."
Avellone said in an interview that Sion was supposed to be an amalgamation of rotten flesh and stone.
__________________ RealistRacism: "Sheevites, much like the Banites, were meant to increase in power with each member. From Lightsnake to Gideon to Azronger, this was supposed to be the case. However, knowledge must've been lost in some kind of Gravid-like incident, as Az turned out to be a mid-tier debater with a sub-par track record, sh!itting all over Tempest's legacy. Sad."