Depending on how excited for a game you are, or how good that game actually is, is what usually determines the amount of time you're willing to put into it. Certain games have so much content that you have no choice but to give up on 100% clearance only because you've beaten the main storyline, pumped 100 hours into it, and haven't even broken 50% of all that there is to do. It's no one's fault. Games do this stuff.
Out in the remainder of the game is usually a boss or a quest so difficult, that you have to have completely mastered the game and gathered all the best weapons to even consider undertaking the challenge. Not everyone has the time for that. And, to be honest, it's not really all that much fun or rewarding. Fighting these super bosses who are much more powerful than the game's true final boss is often pointless. To be that strong, you might as well just finish the game.
Sephiroth can't stand up to Emerald and Ruby Weapon, and Pokémon Trainer Red can wax the Elite Four in no time at all. We get that you want to be the best around the house at a game. But why spend all those extra hours grinding and leveling up without the storyline and mystery to keep you going? It's grueling. But, if you are willing to be a true gamer and push to the very end, there are a ton of optional bosses for you to fight.
25 Sephiroth (Kingdom Hearts)
Everyone knows that this guy was going to be on this list. He's one of the more popular enemies in video game history despite most of you posers never actually defeating him in his original game.
Sephiroth, as a boss, is found in the dark depths of Radiant Gardens. Where he will start attacking almost immediately. He's an endgame boss, and will take a long time to defeat. He has a ton of health.
24 Red (Pokémon Gold And Silver)
Red is basically Ash Ketchum all grown up and super bitter about life. It's why he decides to hang out on the top of a snow-covered mountain looking off into the distance.
This mountain we speak of, and his location, is Mt. Silver.
We know it's Ash. He has a Pikachu, Blastoise, Charizard, Venusaur, Snorlax, and an Espeon. The Espeon is the real kicker. That means Ash had the smarts to level up his Eevee with love into the strongest Eeveeolution.
23 Baron Von Roolenstein (Donkey Kong Country 3)
The entire game, Dixie and Kiddy Kong are looking for Donkey and Diddy Kong. They're trapped somewhere in the Northern Kremisphere. Eventually, it's learned that they're being held captive by KAOS.
The whole way to fight this secret boss is to beat KAOS first. It's at this point that this guy, Roolenstein, reveals himself. He's actually King K. Rool in disguise. And he's the hardest boss in the entire series.
22 Flemeth (Dragon Age Origins)
Flemeth turns into a dragon. She's a shapeshifter, and the best way to defeat a giant dragon is to make yourself as strong against fire attack as possible. This is a common tactic in RPG games.
When you're in the ice village, be strong against ice.
To fight Flemeth, you just need to make sure that when you go challenge her at her camp, Morrigan isn't in your party. If Morrigan is in your party, then you will end up strolling to a campsite that is empty.
21 Adamantoise (Final Fantasy XV)
After you beat the game, you have the option of starting another hunt. Yes, another hunt. Of the thousands of things to do in this game, chances are all you did was hunt. So this is the final one. Your final fantasy.
It's not so much that you need to find Adamantoise, because it's the size of a mountain. Actually, it is a mountain. Or rather, the mountain is the shell of the turtle. The beast is located at Longwythe Peak after the game is completed.
20 Dullahan (Golden Sun)
The way to find this guy is to travel and survive through the games most difficult optional dungeon of Anemos Inner Sanctum. Dullahan is waiting at the end of that dungeon for you.
And he's definitely a handful.
He shows up in multiple games. He's just a guardian of a super important tablet. The Iris summon tablet. It's one of the more powerful items in the game that warrants this sort of guy to protect it.
19 Ruby And Emerald (Final Fantasy VII)
These two guys are pretty popular too. Ruby is pretty easy to find and you might have even stumbled onto him by accident. Big mistake. He lays in the sand surrounding The Golden Saucer.
Just bump into him and fight.
Emerald is a little bit more difficult to find. You need to go underwater in the submarine and find him. But when you do find Emerald Weapon, all you do is crash into him and the battle begins. You're going to lose.
18 Karstaag (Skyrim)
Skyrim has all sorts of difficulty levels you can apply mid game, even mid-battle. But some of the enemies are so tough that even on the lowest difficulties can be a handful. But before you fight Karstaag the Frost Giant, you need his skull.
To find this psychotic fiend, you need to beat your way through Castle Karstaag Ruins. There are a few jerks to fight before him, but once you've made it through the garbage, place that skull on his throne for a battle.
17 Ebony Warrior (Skyrim)
The way to fight this guy is to first and foremost reach level 80. When you're that high up there in level, the next hold you exit or enter, the Ebony Warrior will approach you and issue a challenge.
He's fought many foes, and is looking for a worthy adversary.
He tells you to meet at the Last Vigil. The Last Vigil is a campsite located all the way up in the Velothi Mountains. He'll be waiting for you there. Be ready to fight and make sure you save. This dude is tough.
16 Akuma (Super Street Fighter 2)
Akuma is the famous demon fighter from the series. He's super cool and one of the stronger fighters in the series. But getting to him is not an easy task, and it's reserved for the toughest of gamers.
The way you get to Akuma is by not losing a single battle and defeating all the opponents leading up to Sagat in 20-25 minutes. Do all of that, and you'll be able to face Akuma at the end of the game as the final, final boss.
15 Nameless King (Dark Souls III)
This dude used to be a dragon-slaying god of war. But he decided to sacrifice himself to the Ancient Dragons. Probably due to them being super cute and cuddly. This guy is not worth your time unless you're a real warrior.
Bosses in these games are pretty serious.
Any enemy can give you a game over. But if you're willing to travel up to Archdragon Peak, this dude will be waiting for you. Chances are you will fail a few dozen times.
14 Penance (Final Fantasy X)
This guy takes a bit of doing to defeat. Even to fight him the party has to defeat all of the released Dark Aeons first, who are optional bosses themselves. After all of that gets done, Penance is available for battle.
The place you find him is in the calm lands and when you start the battle, it has to take place on the airship. He simply just floats up there and is a destination for the Airship to travel to. The battle begins when you show up.
13 Cow King (Diablo 2)
The Cow King is one of the super-strong bovine. And obviously, he's some sort of king from the secret cow level in the game. The problem is, he looks pretty much identical to the other bovine.
But Cow King always spawns in a special place.
In a fenced-in enclosure somewhere in the cow level. He's avoidable, but this list isn't about that. We know they are avoidable. But if you get close enough, he will attack.
12 Naydra (Breath Of The Wild)
If you're any good at Zelda games, then you probably won't have much trouble finding this guy, Naydra. He's up on top of Mt. Lanayru. It's an ice mountain, so exploration is best with the right gear.
But if you're like most people, you climb every mountain in this game. Because cool stuff is on the top of mountains. Naydra is infested with Calamity Ganon's stink webs. Clear the bad from this dragon to set it free.
11 Valkyries (God Of War)
The Valkyries are optional bosses that appear late in the game after you've beaten the main storyline. You can travel around the world defeating them for extra stuff, but these Valkyrie are cursed and therefore evil.
Rather than be good servants of Odin and take the perished warriors to Valhalla to hang out and feast all the time, these Valkyrie just want to fight and doom every warrior that comes across their path. Forcing defeated warriors to wander as Draugr for eternity.
10 Culex (Super Mario RPG)
Culex is another guy in this game who happens to be from another dimension who just wants to fight people for fun. That's his thing. The way to get to him is in Monstro Town.
But it's not as simple as opening a door.
Actually, it is a sealed door you have to open. Behind the door is Culex of course, and to open the door, you need the Shiny Stone. Culex will welcome you, tell you his story, and ask you if you're interested in battling him.
9 Magi Master (Final Fantasy VI)
Magi Master is an optional boss who hangs out at the top of the magic tower in the World of Ruin. The whole idea about him being optional is because this battle is unlike any other in the game thus far.
To get into the the tower, you have to fight some cultists. Then run all the way to the top. It'd thousands of stairs. And every battle, your team of warriors can ONLY use magic spells. No physical attacks. It's a doozy.
8 Yiazamat (Final Fantasy XII)
Yiazamat is a super intense boss battle. He's sort of the final "hunt" in the game with a whopping 50 million hit points. You're going to need a team with the best gambit system and weapon on top of a healing strategy to last over an hour.
Yiazamat is found in the Ridorana Cataract.
There's no way you can miss him when you roll up to this spot. He's a giant beast dragon thing. And those weird aura circles are non stop. He lives with those on his face.
7 Morgan Freeman (South Park: The Fractured But Whole)
Morgan Freeman is America's Grandpa. Which is why it's awesome that you can slap him around in the game. He's one of those bosses you can fight at any point, but you're going to get stomped if you aren't at max level and have beaten the story mode.
You can find him at Freeman's tacos.
But this isn't really like a villainous boss. Freeman is a bit of a mentor for the player, teaching them new abilities. He's almost like a warrior sage to finally test your powers gained.
6 Shadow Jago (Killer Instinct)
To get to this guy, you're going to have to go through some garbage to get Shadow Jago to replace Fulgore as the final battle in Arcade Mode. The first requirement, you're going to have to see three endings of a character in Arcade mode.
Then that character needs to beat the game on Hard mode without losing once, get two supreme victories and one ultra before you reach the rival character. Then after all that, you need to Ultra the rival. Which means get an ultra combo on them.
5 Ozma (Final Fantasy IX)
Ozma is considered to be the most powerful boss in the Final Fantasy series. Ozma is the superboss of the game, a sphere of swirling energy, both dark and light. Beating it gets you the Tetra Master Card and the final Eidolon, Ark.
The way you get to Ozma is by completing the game's longest side quest involving the Chocobo forests and the world map, going from forest to forest of unvisitable locations until you finally end up at Ozma's doorstep.
4 Dream Devourer (Chrono Trigger)
So this one is a little tricky. One, because Dream Devourer is not in the original game, but in the DS remake. And two, because you can fight Lavos or his forms at any time in the game past a certain point. But this one is special.
To get to Dream Devourer, an earlier form of Time Devourer, the final boss of Chrono Cross, the sequel, you have to at least have a team of level 60+ or you will have absolutely no chance at winning.
3 Moon Presence (Bloodborne)
The Moon Presence is located in the Hunter's Dream in the game. But to get to him and allow for the fight to actually take place, like most optional bosses, a few things need to be done first and gathered.
Before you get to this guy, you need to consume three out of the four available umbilical cords. Then you refuse Gehrman's quest and defeat Gehrman the Hunter. Make sure you stay mid-range.
2 Yama (Spelunky)
King Yama is located in the last level of the underworld in his lair. It's the last boss of the game. He's not so much optional, but really, if you decide you don't want to beat the game, that makes him optional.
He's tough. He's not only got a bunch of traps and stuff around, he's got his two minions to deal with too. Horse Head and Ox Face. On top of all that, Yama himself is incredibly powerful. After all, he hangs out in the underworld all day.
1 Driviks, The Chosen (Destiny)
Driviks is an optional boss because he's a beast. And defeating him is worth it especially if you're a sniper. The weapon you get for defeating him is the Black Spindle as an attachment or something along those lines for a sniper rifle.
Driviks, The Chosen, is the end of a path that you would take as an alternate ending to a quest. It's the ending of the Lost To Light mission. This is a necessary mission to make if you really want to be a serious warrior in the game.