Lyn Lee Fox
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For reference purposes, this rule can be found on page 15 of the rulebook at the end of the "Fight or Flee" section:
from AH Rulebook, page 15, section 2, subsection B. The rulespeople wrote:
Important: If a monster’s toughness is greater than 1,
partial success has no effect on it. A monster must be
completely defeated in one Combat check or it ignores
all successes the investigator rolled.