He solved the problem solely by himself.

Hitori is my best friend, she always helps me.

Hitori is my best friend.

I went to see a movie by myself.

It takes courage to take a step forward.

She was a secret kunoichi (female ninja).

He threw a handful of sand into the sea.

He missed a once-in-a-lifetime opportunity.

"Once in a lifetime" means to cherish the encounter that happens only once in one's lifetime.

He was focused on his work all day long.