Answer:
tough
Step-by-step explanation:
"Tough" may be formal or informal depending upon the situation in which it is used.
Example:
He was a tough customer to deal. (informal).
He had a really tough time. (formal)
However "onerous" and arduous are very formal words meaning "difficult".
Meaning of "struggle" is not "difficult". It means effort, endeavor, labor. Struggle is a noun, while "difficult", "arduous", and "onerous" are all adjectives (modifying or describing a noun or pronoun).