single-handedly vs single-handed

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

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

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

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

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