This is the 6226th most frequent Russian word.
труднее
"Труднее" means "more difficult" in English.
'Труднее' is again being used as a comparative adjective to describe the relative difficulty between levels.
Любопытно, почему второй уровень оказался труднее первого.
It's curious why the second level turned out to be harder than the first.
'Труднее' describes increasing difficulty over time, comparing it with earlier states of concentration.
Становится труднее сосредоточиться поздно вечером.
It becomes harder to concentrate late in the evening.
Here, 'труднее' is the comparative form of 'трудный' (hard/difficult), used to compare the difficulty of two tasks or situations.