The 8-letter "opposite gender" of a male giant is "giantess."
(There is no common 8-letter antonym to a giant or to the adjective giant.)
enormousgiganticspaciouscolossal
Midnight
deprived
stopping
exciting
Imprison is a word which has the opposite to liberate and has eight letters
stopping :) EASY x
One is shutting.
a. & n. from Start, v.
Indifference is the opposite word of concern.
many, much source: http://thesaurus.reference.com
dismount is the opposite of mount and contains eight letters