These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
YMMV: Mega Man's Christmas Carol
That One Boss General consensus is that Future from the original game/remake, is by far the hardest boss to defeat Buster only.
In the original Flash game, the first fight against Dr. Light is a puzzle boss, and without using the correct solution, was declared somewhere between impossible and incredibly frustrating. Ironically, using his correct weakness, he falls in 3 hits.