Took me about 20 years to figure out how to beat the final boss in Fantasy Zone on the Master System. The final boss is actually a giant Opa-Opa (called Opapa-Opapa) but you don't know it til you beat the game. You just see the tip of the ship, and it shoots 6 giant bullets out at you one at a time. To beat him, you have to destroy each bullet one at a time. The bullets have trails on them, kind of like Snake, and they zip around in set patterns filling the screen. The first 5 are super easy to beat, but the 6th is damn near impossible. Instead of going in a set pattern, it comes straight at you, following you. Even with the fastest speed upgrade, it's faster than you. It takes too many shots to kill before it hits you, even with a cheating rapid fire controller. I thought it was legitimately impossible to beat, until I one day realized that the Heavy Bombs in the game were invulnerable. In the game, the last boss is called "The Assemble" and it's a boss run of every boss in the game. Before the boss run, and every time you die during the final boss, you are given one chance to hit up the shop, and the heavy bombs are the cheapest weapon in the game. If you fly above the bullet exit path of the final boss and drop a heavy bomb as soon as he fires the 6th bullet, it'll kill it instantly before it has a chance to hit you. It's, to my knowledge, the ONLY way to beat the boss.
Once you know that, though, the entire game is a snap. I can loop it several times without dying.
