Translation:I want to leave this hard work for him.
Yes. Although there wasn't an option for wrong/unnatural english translations.
If it ever happens that you don't find a matching tick-box on the drop-down menu, just explain what your problem is in the last box that is open to spontaneous comments.
that's right, because indirect object pronoun "lui" stands for "à+il" or "à+elle"
Since lui is an indirect pronoun, I do not understand why "leave to him" or "leave to her" is not correct.