Irresistible magic ignores magic defense => it also works on Darkness, so go read the manual yourself, you walkthrough enthusiast.

But it's a different matter that I don't particularly respect Lethos. The extremely high spell power that Tier has is much more beneficial to invest in Chaos or Summoning than in Darkness.

And if we return to the topic, then: choice #1 is, of course, Haggeesh. She is effective at any stage of the game. On rush maps, it's Tisleth. In the late game, you can generally take Garuna.