Correct spelling, explanation: single-handedly is a commonly used adverb that is formed from an adjective single-handed. The word dates back to the 18th century and it refers to doing something alone. It is worth paying attention to the spelling of single-handed, which sometimes causes some trouble.

Definition of single-handedly:
1. adverb – alone, without any help,
She single-handedly completed the project, which impressed her boss.

Synonyms of single-handedly:
Some expressions with similar meanings to single-handedly include on your own, all by yourself, and independently:
My daughter finally ties her shoelaces on her own. = My daughter finally ties her shoelaces single-handedly.
My wife is incredible – she painted the whole flat all by herself. = My wife is incredible – she painted the whole flat single-handedly.
It’s high time to start living independently. = It’s high time to start living single-handedly.

Incorrect spelling, explanation: single-handedly is an English adverb, which is often misspelled as single-handily. The mistake probably comes from the fact that the word handily itself is correct. However, its meaning is completely different from single-handedly and there is no such form as single-handily.