1. Removing their own brain and sticking it in a jar full of preservatives. A replacement made of glass, vital fluids, and clockwork ticks behind their eyes now, letting them control their body from afar. They can rebuild the whole thing if it’s destroyed, though it would be costly in time and magical resources.
2. Marrying a fairy prince. So long as the wizard wears their wedding band on their finger, they won’t die until their spouse does. They locked up their spouse in a cage outside the normal flow of time to keep them secure after consummating the marriage.
3. Converting their thoughts into noospheric shockwaves. Sensitive individuals dream of the wizard, living their life while asleep and gradually overwriting their selves with the wizard's own. Wizard-duplicates can’t abide each other’s presence due to an extreme case of narcissism of small differences.
4. Grafting magical hybrid plants to a dryad’s tree then absorbing the dryad to take on the lifespan of their now-eternal tree.
5. Regularly bathing in volatile alchemical concoctions. The baths rejuvenate them, but each time minor mutations accumulate, and each subsequent bath requires rarer and rarer ingredients.
6. Gradually replacing parts of their body with far more durable artificial replacements.
7. Stitching a magic cloak from the still-living skins of newborn lambs. So long as they remain totally covered by the cloak, they’re hidden from death’s sight and can’t be taken by it.
8. Arranging carefully balanced bargains for their soul with many powerful demons. Until the demons resolve who has the strongest claim on the wizard's soul, they won’t let the wizard die and start a free-for-all.
9. Preserving the most important parts of their self within an enchanted amulet while offering the rest to an alien spirit. The spirit animates and provides the basic drives of their body, while the wizard retains executive control so long as the body wears the amulet.
10. Blaspheming the rites of a mother-goddess. Now they can give birth to themself and live on through the generations. Conducting one’s own rebirth is a taxing physical and magical exercise though, and the reborn wizard must consume the birthing wizard immediately after or else wither away.
11. Temporally tying themself to an hourglass. While the hourglass is running one way they age normally in time with it. When the hourglass is flipped they age backwards in the same manner. Should the hourglass run out in either direction they’ll die. Should the hourglass be shattered the wizard will explode into a local time-rift.
12. Writing an autobiography that described their self so totally that the text became more real than their physical self. Destroying their body is only an impediment until the book can recreate them. Destroying the book renders them mortal until they can rewrite it. The only ways to put them down for good are to destroy both simultaneously, or to cause the wizard to diverge from their description in the book significantly enough to cause a break in the narrative reality.
13. Establishing a cult that worships them as an aspect of a god, and using that adoration as a conduit to sip at divinity. So long as the worship continues, so too will the wizard.
14. Killing the very possibility of their death. They possess no special durability though, and have only continued to age and shrivel with time. They will not die even as the passing of epochs (or adventurers) grinds them to dust.
15. Snarling the timestream and drawing a clutch of their younger selves as apprentices. When they grow old they transfer their mind into their most promising apprentice-self and kill the rest. Through temporal shenanigans the apprentice-selves are starting to cotton on to their master-self’s fatal designs.
16. Halting apoptosis and entering a perpetual adolescence. They are an excrescent giant, swarmed by oncologist-mites that burrow through their organs and devour the tumours that swell within.
17. Hiding their heart within a swarm of flies within a pair of hummingbirds within a gazelle locked up in their ribcage. When their body dies the gazelle will leap from their mouth. When the gazelle dies the hummingbirds will fly from its ears. When the hummingbirds die the flies will flee from behind their eyes. If any of the animals escape, they’ll be able to regrow the entire wizard, slowly but surely.
18. Locking themself in a modified torture device they stole from hell itself. It keeps them alive even as it torments them. They require a constant supply of supernaturally potent drugs to stave off the agony, and have attached an animate exoskeleton to the device so they can still move around. Should their supply run low the pain would surely render them nonfunctional.
19. Shedding their meat and becoming a being of light and lightning. They require special interfaces (or just their own prodigious magic) to interact with the solid world, and can only be harmed by esoteric means.
20. Sealing themself inside a perfectly sterile, self-sustaining arcology. So long as the arcology remains untainted they’ll be able to remain immaculate forever. Their simplified composition would be quickly devoured by external bacteria however. They interact with the outside world through projections and possessed proxies.