Yup, life for a life. Two casts should do it, unless you're unlucky.
I think you're risking blood vengeance with any of the hp spells... and probably the hp-based instadeath spells (isn't Soul Slay a 999 hp attack? Which is usually death, unless you're an in-dominion Gift of Health'd Sphinx or maybe a well-fed Eater of the Dead), too. Basically you want decent MR because of both the blood vengeance and the MR-negates death attacks.
The Tartarians, being former deities, also have good MR so long as you get rid of the frequent Feeblemind.
Haven't yet checked whether a failed MR vs. Blood Vengeance roll on a Banish to Inferno will send the caster to Hell... which would be cruelly funny, IMHO.
