How to Make a Final Boss

This is probably the most blatantly obvious but important step: Make sure the final boss has a major role in the story! Quite frankly the whole main villain=final boss is cliched but not overused. It's the best strategy to go with. The final boss is typically the cause of the protagonist having to go through so much trouble, so show that.

Make sure it's hard. Something that many sites categorize as a disappointing final boss is a boss who can be killed in one or two tries. Try to make your boss be so powerful and such that he/she can't be killed in the first three tries. Make the final boss merciless, if possible.

Try to make it epic! If you think about it through and through the final boss fight is the exact same thing as a climax except for video games instead of film. Try to have the regular stuff: dark skies, lightning, rain, etc. Or alternatively, if your game is a more kid-friendly game, don't do that and go for something that will get the E rating (if you're being rated) and make it easy on the feelings, however still make it epic enough to get the E rating.

Have music that is very catchy and all but impossible not to like. Many video games nowadays have the tendency to make the final boss music a remix of the game's main theme but a much darker tone. This is a good strategy but be creative, or not do this at all and make an entirely different track.

Make sure their health is high compared to the other bosses (except for optional bosses and bosses that you are supposed to deliberately lose against, although optional bosses can be weaker than the final boss).

While the boss should be very hard, make sure that they are beatable. The prime example of a game that does not obey this rule is Mike Tyson's Punch-Out!! where the eponymous final boss is commonly cited as one of the, if not the, hardest final boss(es) in video game history.

Get the right instruments. For music the most commonly-used instruments for a final boss is an organ and/or opera; this isn't a must however. In the video game Banjo-Kazooie the final boss' music was very catchy and a remix of the hub world theme, but rather Grant Kirkhope (the game's music composer) used just about every instrument but organ and opera. However, feel free to do what you want so long as it ain't copyright infringement.

Try to see how hard the final boss is without various items that are in the arena. If it's pretty hard no matter what then try to put a few must-need collectibles hidden in the arena and perhaps even a few health items. If it's pretty easy with these collectibles then by all means omit them to make the final boss all the more challenging. A save point and a few treasure chests is a must, especially in the RPG genre.