[3.5] How balanced are Thri-Keen?

[Alteran]

Enervation + save spell is good, but by no means actually fits the "casters kill everything in one round" complaint.

Not at all. As a 4th level spell, Enervation is a prime candidate for metamagic abuse. Arcane Thesis (Enervation) combined with Easy Metamagic (Twin Spell) lets you have a Twinned Split Empowered Enervation as a 9th level spell. That's 4 rays, each dealing 1d4 * 1.5 negative levels. That averages out to 15 negative levels if they all hit. With a Greater Rod of Maximize Spell or Sudden Maximize, it's 16 + 2d4, or an average of 21 negative levels. That kills anything with 21 or fewer HD without a save, and it can be done as low as level 17.

Edit: I was wrong, you can actually have a Twinned Split Maximzed Enervation as a 9th level spell. With a Greater Rod of Empower Spell or Sudden Empower that gets the same end result, but if you buy the rod then you're spending less money. You also inflict 16 negative levels with just the base spell, so along with a Quickened Maximized Enervation (spell level 9) you can kill anything with 20 HD or fewer, no items required.