Single-handed means done entirely alone, with help from nobody. One person, start to finish.
It works as an adjective and an adverb, so a single-handed effort and sailing single-handed. The sailing sense is the literal origin, and single-handed ocean races are among the most demanding events in any sport. There is a medical practice sense too, since a single-handed GP practice has one doctor. The adverb single-handedly means the same and is commoner in ordinary speech.
1adjectivesingle-handedDone by one person alone, without help.
“It was a single-handed effort from beginning to end.”
Also accepted in games: single-handedly
single-hand Back-formation from single-handed.
Etymology from Wiktionary, CC BY-SA