"Doing... is difficult" 可以用 "It is difficult to do..." 或者 "Doing... is challenging" 来表达。
来源：difficult 是形容词，表示 "困难的"；challenging 是形容词，表示 "具有挑战性的"。
例句：
It is difficult to cpmb that mountain. (爬那座山很困难。)
Learning a new language is challenging. (学习一门新语言具有挑战性。)