Boss Vulnerability

Boss Battles in video games can require a wide variety of strategies and tactics to defeat. However, how vulnerable they are to the player characters' attacks tends to vary from game to game. There are generally four types of boss vulnerability:

Always Vulnerable The boss is susceptible to damage at all times, and usually attacks relentlessly. This type can be difficult due to the fact that the player must concentrate on both avoiding the boss' attacks and hurting the boss at the same time.

May be a Damage Sponge Boss. May or may not receive Mercy Invincibility when you hit it, depending on the genre.

Wait Them Out The boss cannot be harmed by any means until a certain amount of time has passed or a certain action is performed, at which point they will become vulnerable for a short period of time. This type can be easier due to the fact that you only have to concentrate on survival, and not have to balance offense and defense. However, it can be annoying if you miss the window of opportunity, since you will have to go through the attack pattern again before getting a chance at another shot.

Attack Its Weak Point and Tactical Suicide Boss are common tropes for this type.

Mixed A mix of the above. The boss is technically vulnerable at all times, but can become stunned or otherwise more susceptible to the player character's attacks. This may happen after a certain time has elapsed, or a special action or actions may be required.

Miscellaneous Any boss that doesn't fit into any of the above categories. These are likely to be Sheathe Your Sword battles, Puzzle Bosses, or some other type of atypical battle.

A Sub-Trope of Boss Battle. Bosses that are never vulnerable are a Hopeless Boss Fight.

Examples of Boss Vulnerability include:

Always Vulnerable

Most bosses in the Castlevania games.
Most Role-Playing Games use this type by their very nature, though other types do crop up sometimes.
Distorted Travesty.
Every boss in Iji, except the final one which is mixed, and Iosa's second form, which must be waited out.
Most bosses in the Kingdom Hearts series are either this or mixed.
Most bosses in Hero Core.
Ridley and Mother Brain in the Metroid games.
Devil May Cry.
Every boss in Eternal Daughter except for the final one, which must be waited out.
- The Final Boss requires you to go through a short but difficult series of actions in order to make him vulnerable, at which point you can hit him once before having to make him vulnerable again. It's a long fight.
Most bosses in La-Mulana.
Most bosses in Cave Story.
The most common boss-type in the Mega Man series. Almost every boss falls into this, and the ones that don't usually need to be waited out.
The second phase of the fight with Skeldritch in The Legend of Zelda Spirit Tracks.

Wait Them Out

Most bosses in the Mario games.
An Untitled Story.
The Liquid Metal Processor and the final form of the Cruiser Tetron in Hero Core.
Final Fantasy XIII has a few bosses (and regular enemies) that are practically invulnerable until they are staggered.
- Lots of Final Fantasy XIII bosses, though after you max out the crystarium, many of the mission marks go down faster if you just go all out instead of trying to work out a stagger.
Most bosses in the Metroid games.
The Cloud Child in The Spirit Engine 2 has a separate life bar that acts as a shield. Once you reduce it to zero, the cloud child will be stunned until the shield regenerates to full, during which you can attack its main life bar.
- The first form of the Final Boss is also invincible until a certain amount of time has passed. Its second and third forms are also difficult to damage until you destroy at least one of its flunkies, since they'll block your hits otherwise.
Every boss in Lyle in Cube Sector, except for the semifinal one, which is always vulnerable.
The Final Boss of Eternal Daughter.
In Persona 4, Shadow Rise cannot be damaged, but halfway through the fight, she leaves and you end up fighting Shadow Teddie instead.
Most bosses in the Sonic the Hedgehog games, though there are a few bosses that subvert it.
Most in I Wanna Be the Guy, although some sections and bosses are Type 1. Thankfully, their vulnerable sections tend to be frequent.
A few bosses in Mega Man fall into this category, notably ones that can fly out of your reach such as Gyro Man in Mega Man 5 (unless you have Gravity Hold).
The Batomys in Valkyria Chronicles is only vulnerable the round immediately after it's fired its main cannon and exposes its radiators to cool down.
Most bosses in the Zelda series are this, with most of the exceptions being in the first two games (and the aforementioned Skeldritch round 2), which are always vulnerable. Typically, you have to use the item you just got in the current dungeon to make the boss vulnerable to your sword.

Mixed

Most bosses in the Kingdom Hearts series are either this or always vulnerable.
The Final Boss of Iji is technically vulnerable to most of your weapons, though even the MPFB Devastator is not going to get you very far. The most effective way to injure him is to wait out his attacks until he uses a powerful Charged Attack, then reflect it back at him.
Pinstripe Potoro in Crash Bandicoot is a mix. He's vulnerable at any point when not firing his gun (and technically vulnerable before then but you'd get torn to pieces trying), but only for the first three of six hits. For the last three, he becomes invincible, and you have to wait for his gun to jam.
Corbenik in Project .hack. Despite having glitched/"infinite" HP, your attacks still deal normal damage. In reality, all three of its forms have 7000 HP. The second one will put up Ultimate Defense partway through the battle, which makes him invincible until the plot kicks in.
Ghirahim from The Legend of Zelda: Skyward Sword. While he's not exactly immune to any of your attacks, he's a master swordsman and can easily block slow or sloppy attacks. You'll have to be smart about how you swing your Wiimote in order to get past his defenses. This gets subverted somewhat for your final battle with Ghirahim; the first phase is a Ring Out Boss, while the second phase is type 2.
In Viewtiful Joe, some bosses are vulnerable at all time (usually early bosses), some are only vulnerable after certain conditions (such as Fire Leo), and some are perpetually vulnerable but can be stunned for a longer combination of attacks.

Miscellaneous

GLaDOS in Portal, who is also a Puzzle Boss.
Dark Cecil from Final Fantasy IV. Kind of a given, since it's the Trope Namer for Sheathe Your Sword.
The tentacles in Half Life. They are a Puzzle Boss immune to weapons that takes some running and turning on fuel and electricity to defeat.
Every Phase except Corbenik, which is mixed instead in Project .hack. The bosses take damage just fine—their HP count is just glitched up, making them undefeatable. Data Draining them fixes this and also causes them to lose a lot of their special attacks, including their own Data Drain.
A rather odd variation in the second Dark Cloud game: The True Final Boss, Dark Element, changes its color between red and blue every so often during your fight. When it's red, only Max can hurt it. When it's blue, only Monica can hurt it. Unfortunately, it's never vulnerable to the Ridepod or its Nova Cannon.
The Grand Mother from Hero Core is a bit of an odd example. It's completely invincible until you destroy its flunkies a few times, at which point it goes berserk and loses its invincibility entirely.