[Will]

If you're attacking with Hydras, and using Foul Vapors, I would assume you know to have 100% poison resistance on your mages and bodyguards...

As for aging, I think having nature magic makes your max age higher, and death magic gives a reduced chance of afflictions once old age is reached.

I wouldn't bother with empowering in nature for this particular crone. Quick manual check, looks like the only battlefield spells that would give benefit are Personal Regeneration (it says 10% +(Nature magic * 100/MaxHP), which could be close to 100% for a N9 crone... but a one-round kill of a Crone is not too difficult), Sleep Cloud (bigger AoE), Storm of Thorns (more vine arrows), Panic (bigger AoE), Creeping Doom (more ants... scary! not), Polymorph (make more enemies into bacon), Stream of Life (bigger chance of killing instead of making a fully healed berserker), Healing Mists (bigger AoE) and Wild Growth (more tangle vines). I don't think any of that is really worth gem cost of empowerment. The only battlefield nature spell out of reach is Gaia's Blessing, which requires earth anyway. For summoning, you can get a Tarrasque with just a Thistle Mace, and can get Faerie Court and the Treelords with no buffs.