For 16 years now, Adam Dawes has had a guide to Bubble Bobble on his website that provides precise, detailed strategies for defeating each of the game’s 100 levels, most with a demonstration video (one level’s video I found doesn’t work). Each level has a difficulty ranking, and such is skill that the hardest of them I found, level 91, is only rated as “medium-hard.”
Adam’s guide provides the details of finishing each specific level, but it doesn’t explain all of the weird secrets that lie buried deep in the game’s code. For that, check out the previously-linked Bubble Bobble Info Pages!
Adam Dawes’ Guide to Bubble Bobble