Stage 3, all of it.
I get trapped very easily during the whole stage.
mid-boss is already too hard, i won't even mention the boss.
it's really a matter of practicing more, really, but i can't be bothered because of all the loading times.
I can constantly no bomb/no miss the first 2 stages, btw. So it's also a case of good bomb management, i could get to stage 4 more often.