More LA R'lyeh and spellcasting questions

[Aleph]

Why use it instead of Soul Slay? Because I specifically stipulated using the strategy against immortals. If they are horror marked enough, their life will become 1. Take 1 step away from capital 2. Get killed by horror, increase horror marking. 3. Wake up in capital. 4. Repeat with larger horror.

I'll admit, it's hard to do. But the OP was asking about when Dreams is better than Mind Hunt IF you know your opponent doesn't have astral mages (quick aside - if you know your opponent doesn't have astral mages, you can also be pretty such he doesn't have an antimagic amulet). This is one such situation.