I suppose, and yeah, AoE energy damage would work....though the flaming melee weapon would still need to hit the stump's AC, so it's relevant. They can't just stab me in the belly and seal a bleeding head.
And I promise I won't make myself immune to both fire and acid, because that's cheesy and makes me unkillable except though instant-death magic. Immune to one and resistant to the other is as high as I'll go.
Planned build this time...
Mystic Path: Totemist 12 (soulmelds are supernatural, but they are not spellcasting).
Martial: Swordsage 2/Hexblade 4/Soul Eater 6. Everything is either Skilled or full-BAB, I multiclass once and have entered a PrC that will continue to completion, and has my single spellcasting class.
Monstrous: Seven-Headed Pyrohydra (7 HD, CR8) with the Lernaean Multiheaded Creature x1 template (+4 LA).
Verify before I start crunching numbers this time?