Balm of Chésmè
Seventh level Fyvrian Spell. SB probably somewhere around 10-12 if it's primary convocation, lower if not. So when you first open the spell, your ML is maybe 20-24.
You can only cast this spell on yourself until ML 86. So how to get it there. General rule, just keep practicing casting it until you get it up there. But... it "should not be laid on the same subject more than once", because it'll probably kill you sooner rather than later.
There's a caveat: "Dispelling the enchantment removes the interactive residue." Ordinarily, I would think that means while it's still in its duration (four months for a marginal success, one year for a critical success). But then, at ML 91+ "No dangerous residue is left, even with MS". Which says to me that casting dispell after the duration ends should work. And at ML 91+ you no longer need to do that.
All this leads me to: learn 7th level Dispell (neutral, multi-level) so that you can dispell this thing each time you "practice" it. If you cast Balm, get MS (say), and don't wait for four months to dispell it, you won't get much longevity from it, but then your goal is to increase the spell ML; increasing your life span is secondary.
But wait, there's more! :-)
"ML96+ With CS, the caster may cast the spell a second time on the same subject without special risk to the subject."
*That* implies that below ML 96, even with dispell, you still have the "special risks".
This seems like a prohibitively tough spell to learn to a reasonable ML. How would *you* handle it?