"boundless determination" is correct and usable in written English.
You can use it to express an immense amount of determination, usually regarding a specific goal or purpose. For example: "With boundless determination, he worked diligently to achieve his desired outcome."
This plucky redhead, in Lucy Maud Montgomery's Anne of Green Gables, is known for her boundless imagination and determination not to be bested.
What I found most interesting about him, beyond his charisma and intelligence, was his boundless curiosity, his determination to bring progressive ideas to the four corners of the universe.
There are so many people with boundless enthusiasm and determination who are doing research, who beam when they explain their research, and who are passing on knowledge to a new generation.
She lived life to the fullest, inspiring each of us with her optimistic outlook, boundless energy and steely determination.
"It is with humility, determination and boundless confidence in America's promise," she said over a roaring crowd, "that I accept your nomination for President of the United States".
"So it is with humility, determination and boundless confidence in America's promise that I accept your nomination for president of the United States," said Hillary Clinton – the first American to stand on the brink of being called madame president.
For all the talk of how nothing could be done alone, Hillary Clinton had one singular duty Thursday, which she performed early in the speech: actually accepting the nomination — "And so, my friends, it is with humility, determination, and boundless confidence in America's promise that I accept your nomination".
"It is with humility, determination, and boundless confidence in America's promise I accept your nomination for the president of the United States," Clinton told the crowd as she officially became the first woman to lead a major political party's ticket.
That's the future we're working toward … And so it is with humility... determination... and boundless confidence in America's promise … that I accept your nomination for president of the United States!
Mae's love and determination, with her boundless support for her family and their endeavors, really was the wind beneath their wings.
