Innsmouth does seem to be drawing these out of the woodwork, though this one was possible - though nowhere near as easy - since CotDP.
Ithaqua is really not in good shape vs spell casters with a Heal spell that manages to survive the equipment destruction who have some way to offset the sanity cost (Agnes can pay it out of stamina, and will generally get 2-3 back per casting; Harvey can pass their PS and similarly regain stamina fast; Dexter likewise, and Daisy starts out that way).
Harvey can pull the same trick vs Yig, even more easily, at least once the Curse is removed.
The Feeding the Mind spell from CotDP, of course, lets you convert 1 stamina to 2 sanity. You can then use one of those sanity to cast Heal and get several back. Again, it's just a useful combination during the game, but Yig stands no chance vs a strong spellcaster with those two spells.
It's not too bad - you have to obtain the spells, and either clear the Curse quickly (Yig) or have them survive into the fight (Ithaqua), so it's by no means as guaranteed as Roland vs Nyarlathotep or Michael vs uncorrected Rhan.
Hastur vs Feeding the Mind+Heal, on the other hand: a Lore 6 character can on average spend 2 stamina for 4 sanity. They can then on average regain 2 stamina from Heal at a cost of 1 sanity, lose 2 more from Hastur's attack, and still come out in profit. Harvey has a much easier time - with a passed PS, Heal doesn't have a sanity cost, and Hastur only does one damage, so only a single success (on a 7 die Lore check) is needed for each spell. Dunwich's weaker Cloud Memory spell would be sufficient here.
(and yes, I know, Epic Battle fixes these)
and Hastur will lose.
.
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