These rewritten heroic-historys are largely worthless.
Kim Il Sung participated in two attacks on the Japanese in the Korean Peninsula confirmed by modern historians (at Hyesan and Pochonbo, the latter of which may have been led by someone else) and other attacks in collaboration with the Chinese communists (see book above for details).
For those wondering why Korean nationalists would have become collaborators: they thought colonozation by an Asian race was less likely to erase Korean culture than colonization from a white race (as the nationalists believed a Japanese defeat in the pacific war would lead to Western colonization of East Asia)
culturally, in many ways it did
The Communists used guerilla tactics, to much success in some cases. Who did more to stifle the Japanese war effort is an impossible question to answer, but both sides of the Chinese Civil war contributed in the fight, each to their own strengths.