Boss Game

Some video games feature a Boss Rush, which is when you have to fight several bosses in short succession. Boss Games take this to the next level: The whole thing is nothing but boss battles with sometimes the occasional breather segment in between. The polar opposite is Mooks but No Bosses, and the extreme end of Easy Levels, Hard Bosses. Boss-Only Level is a Sub-Trope, in which only one level is just a boss.

Boss Games come in three flavors:

An original game with a main focus on fighting bosses: There may be "fodder" enemies (or at least pauses) between each fight, but those segments are easy and very short.
The entire game is one continuous battle against a single opponent, usually one who changes depending on the performance of the player. These games tend to be rather short but intense, and are almost always 2D Shoot-Em-Ups.
A "special" edition/port/hack of a normal game with everything except the bosses removed, similar to a Boss Rush.

Examples of Boss Game include:

Type 1

Alien Soldier. There are more bosses than levels if you count each form of Seven Force separately!
Contra: Hard Corps and Shattered Soldier.
Aero Fighters 3
Shadow of the Colossus - interesting because they are all Puzzle Bosses, and, appropriately, Colossus Climbs.
Radiant Silvergun - In some cases, there's only a short segment of normal enemies between bosses and after stage 5, there are no normal enemy segments between bosses.
King of the Monsters 2 (The Neo Geo original, the SNES version has longer levels and the Sega Genesis port is more of a straight one-on-one fighter.)
The Windows Touhou games are loose examples of type 1. The stages aren't exactly short due to fixed scrolling rate, but depending on the difficulty boss fights can take five to six times as long as the stage (or more than ten times if the bosses have attacks that render them invincible for a duration). The PC-98 games had better developed stages and don't fit this trope as well.
- Special mention goes to the 9th game, Phantasmagoria of Flower View, which is 100% boss fight, with random Mooks flying around in order to allow you to build up your Spell Cards and attack your opponent.
Stretch Panic
No More Heroes. Especially the sequel, where everything but the boss battles (and the short levels leading to them) became optional.
Mega Man: The Power Battle and the sequel The Power Fighters are Boss Games of this type, masquerading as fighting games.
Chaos Field. The original game consisted entirely of boss battles, while the Expanded mode in the Gamecube version has waves of cannon fodder enemies between bosses.
The Monster Hunter series.
Blood Will Tell
Fraxy. You have a choice of either choosing what boss you wish to fight, or letting the game choose for you. Be warned, however, that the game will sometimes pit you against That One Boss.
Every Fighting Game is basically this. This is especially noticeable with older games like the original Street Fighter or the first Fatal Fury, which had a much more limited choice of player characters.
- The first Fatal Fury was very obviously an action game with a fighting game setup (one punch, kick, and throw button, clear demarcation between the heroes and the enemies, 2-against-1 mode).
- Monster Maulers, Metamoqester and Red Earth/Warzard are rare examples of boss-based fighting games made after Street Fighter II.
Forbidden Forest and Beyond Forbidden Forest for the Commodore64. A relatively short game which is more or less a Boss Game, the second more so than the first. Notable for the fact that you play as a Bounty Hunter who's been paid to make a hit on a god. Yowza.
The Arena game in Kirby Super Star, and the Helper to Hero and True Arena games in the Updated Rerelease Kirby Super Star Ultra. In the second of the three, you play not as Kirby, but as a helper-fied mook.
Clean Asia
The flash game Bosses!. (One of them is Mega Man in all but name, and you in fact play as a Mega Man Expy with the same attacks.)
Banana Nababa is a throwback to 8-bit, Nintendo Hard bosses. Mercifully, if you die you only have to repeat the boss you died on and not lose your entire progress.
Gundemonium Series (Recollection) and GundeadliGne
The Punch-Out!! series, essentially boss-only vertical scroll shooters with fists instead of bullets.
Creature Shock is a Full Motion Video example of this. After the Rail Shooter opening, the whole game consists of a simplistic adventure game broken up by light-gun fights against alien creatures, all of them completely unique.
Kageki. Arcade was bosses-only; Genesis port had a few token mooks. Interestingly, the arcade cabinet made a half-baked attempt to pass it off as a boxing game with "Three knockdowns = TKO (Technical Knock Out)", this despite the fact that only three foes in the game require that number of knockdowns.
Yie Ar Kung-Fu. The hero (Oolong) was a little bitty sprite who used a bunch of chopsocky moves against a series of increasingly tougher opponents, also little bitty sprites. The reason the game required so much empty space above the combatants was that Oolong could jump about 40 feet high, and in fact lots (and lots and lots and lots) of jumping was key to beating most of the opponents.
Power Stone 2, especially the Pharaoh Walker and Dr. Erode fights.
Urban Reign. There are a few characters that qualify as flunkies, but for the most part, you're up against various combinations of big bosses, lieutenants, Elite Mooks, Quirky Miniboss Squads, and the occasional Worf. Many of the stages allow you to have a partner.
Endless Frontier and its sequel may be considered this. The mooks are mostly weak and easy to go through, while bosses take a while and there's tons of them (And often you face them twice). The sequel even has hunting a bunch of Bonus Bosses as a Sidequest.
Street Fighter 2010, despite its name, is a platformer (that also has nothing to do with Street Fighter) with very short stages. The meat of the game is the boss fights, and Capcom knew it. Many bosses don't even have a stage preceding them, and you're immediately thrust into the fight.
Krazy Ivan. There are randomly-spawning mooks, but most of the game is spend in one-on-one shootouts against unique enemy mechs.
r Rootage. 30 non-random stages * 5 bosses per stage * 4 modes = 600 boss battles.
Battle Clash and its sequel are light gun games that consist entirely of boss battles with Humongous Mecha.
EverQuest 2, especially when it comes to the raid dungeons, has been getting steadily more like this.
Castlevania Fighter, a homebrewed game developed using MUGEN, is a humongous boss rush where you choose a character, choose a difficulty level, then take on just about every meaningful boss from the series that has a sprite which wouldn't clash with those of SotN-styled characters. Oh, and most of them have even more attacks than they did in their original games.
Death Duel. Notable because it's an early Light Gun Game (... without the light gun).
Strider 2. There's actually a wide variety of fodder enemies, but the levels are very short and often end with a mid-boss battle.
Bomberman Quest. Every enemy is a miniboss with some HP, different attacks, weaknesses and a battle theme playing while they're not yet defeated.
Star Wars Episode III. The levels get shorter and shorter, and increasingly focus on you vs. one enemy, to the point of being a Fighting Game with a few short hallways between arenas.
Titan Souls takes place in a mostly empty dungeon except for boss rooms, every boss is a One-Hit-Point Wonder that needs to have its weak point exposed generally by using an exploitable flaw in its attack patterns (you only have one HP as well so avoiding attacks is mandatory).

Type 2

The flash game Level Up!
Warning Forever
Ultimate Crab Battle
A game on Neopets is appropriately called 'The Neverending Boss Battle'.
The obscure arcade game Omega Fighter consists entirely of a battle against a giant enemy warship, with each stage corresponding to a different part of the ship.
The indie Xbox360 game You Will Die
You Have to Burn The Rope. This one is a major nutbuster.
The old Vector Game Star Castle, though from an era when it was not common for video games to have levels to explore or varied stage design, has the one big enemy to destroy always present.

Type 3

Darius Alpha, a rare variant of Darius Plus in which you fight all of the bosses of Darius Plus one after the other.
Ketsui: Death Label on the Nintendo DS (with the "Extra Course" being the sole exception by virtue of being a full-length stage with a special version the game's True Final Boss at the end of it).
- Same for Do Don Pachi Daioujou: Death Label. At the end, you fight two Hibachis at once.
Touhou 9.5: Shoot the Bullet is a straighter example. Since the objective is to take photos of the various residents of Gensokyo, each stage consists solely of Aya vs Boss. Also true of its sequel, Double Spoiler.
The Sonic the Hedgehog 2 ROM hack Robotnik's Revenge is a boss rush of all 17 bosses from the first two Sonic the Hedgehog games.
A cheat code for Kirby's Pinball Land will turn it into this, eliminating the main pinball stages and instead sending you straight to the boss battles.
There are two different passwords in Xexyz that allow you to play against only the bosses (one for the odd-numbered ground stages, and another for the even-numbered flying stages).
The Mega Man 2 fan game Rockman 2 Neta, which allows you to fight the 8 Robot Masters of said game all at once.
Reallyjoel's Dad mode in Hero Core is a parody of this that's supposed to be impossible to beat. It consists of a single room that contains every single boss in the game.
- Well, almost every boss.
The Super Mario World ROM hack Brutal Mario/Super Kituku World by Carol
Most of the later Ys games have a "time attack" mode where you fight all the game's bosses one after another.