This is the 6262nd most frequent Arabic word.
أصعب
"أصعب" means "hardest" or "most difficult" in English.
In this sentence, 'أصعب' is used as a qualitative adjective in the superlative form, describing the most difficult challenge.
كان تسلق الجبل أصعب تحدٍ واجهته هذا العام.
Climbing the mountain was the hardest challenge I faced this year.
Here, 'أصعب' is used comparatively to indicate that learning Japanese was harder when compared to the speaker's expectations.
وجدت أن تعلم اللغة اليابانية أصعب مما توقعت.
I found learning Japanese to be harder than I expected.
In this usage, 'أصعب' compares two methodologies, stating that one was more difficult than the other for the students.