single-handed vs single-handedly

single-handed is an adjective but single-handedly is not an adjective.

Word	Noun	Adjective	Verb	Adverb
single-handed	No	Yes	No	Yes
single-handedly	No	No	No	Yes

As adverbs, single-handed and single-handedly are synonyms defined as:

single-handed (adjective)	single-handedly (adjective)
without help from others
unsupported by other people

single-handed (adverb)	single-handedly (adverb)
without assistance	without assistance