heroic has 8 distinct synonyms spread across 5 of its 6 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
very imposing or impressive; surpassing the ordinary (especially in size or scale)
“an epic voyage”
having or displaying qualities appropriate for heroes
“the heroic attack on the beaches of Normandy”
showing extreme courage; especially of actions courageously undertaken in desperation as a last resort
“made a last desperate attempt to reach the climber”
a verse form suited to the treatment of heroic or elevated themes; dactylic hexameter or iambic pentameter
1 of the 6 recorded senses carries no synonym, antonym or related term, so it is not shown above. Every sense, with its definition and examples, is on the full entry for heroic.
Synonyms, antonyms and the broader and narrower relations for heroic come from Princeton University’s WordNet. Full sources and licences.