strongly has 1 distinct synonym spread across 1 of its 2 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. Its one recorded opposite is weakly.
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.
with strength or in a strong manner
“argues very strongly for his proposal”
in a powerful manner
“the federal government replaced the powerfully pro-settler Sir Godfrey Huggins with the even tougher and more determined ex-trade unionist”
Synonyms, antonyms and the broader and narrower relations for strongly come from Princeton University’s WordNet. Full sources and licences.