Help With DIY Emerald Legion: the Immortal Do-Nothing

[Doctor Despair]

I'm slightly unclear on what the goal is here. For example, are body-swapping tactics allowed? Initiate of Mystra? Planar traits?

Essentially, to make a character that is functionally immortal -- ideally while still being a remotely playable concept.

For example, a Dragonborn Telthor Initiate of Mystra using Selective Spell[you] Antimagic Field is immune to all magic-source damage and all mundane source damage leaving only a handful of damage sources which count as magical while working in an AMF (Serrenwood, Soulknife, Orb of Force). It's available at ECL 11 in persistomancer form with level buyoff. Maybe add in Starmantle and Evasion (via Spelldancer) while keeping Forceward handy to finish off vulnerabilities.

Telthor is interesting. As you said, the above build is vulnerable to Serrenwood archers, Soulknives, and prismatic wall/spheres, as well as other Initiates of Mystra/Invoke Magic users.

Making the build in the OP incorporeal should grant a 50% miss chance to corporeal fire spells, so that's something else to consider, I suppose. 50% miss chance, 50% damage -- although the damage on postmen will probably still be enough to overwhelm him. :/