Which four areas do TOEFL test and which ones require creativity?

TOEFL tests reading, listening, writing and speaking. Writing and speaking require students' creative responses.

What are some difficulties that non-Korean speakers struggle with the most when learning Korean?

From my experiences of teaching Korean, students tend to have hard times learning to distinguish between formal and informal tone, to form the sentence order correctly and to memorize the pronunciation irregularities.

How would you approach teaching a student who strives to learn English as his/her second language?

I would ask the student about his or her hobbies or interests and start discussing these areas with students in English so that they could learn the language passionately.