Vampire Killer/Bosses

As Vampire Killer is a reimagining of Castlevania, all the bosses in the MSX2 game are modified versions of their NES counterparts. Something important to note is that every boss in Vampire Killer has a cap on the max amount of damage they can take per hit from any of Simon's Primary Weapons - no matter what main weapon you hit them with, they won't take more than 1.5 points of damage at a time.

Vampire Bat
The Block 1 boss in Vampire Killer is far more of a threat than its NES counterpart, due to its fast movement patterns and ability to shoot fireballs. The lack of a substitute for Castlevania's Axe sub-weapon in Vampire Killer (i.e., a weapon that arcs upwards in the air) also makes this fight more challenging.

The best strategy for the Vampire Bat is to grab the Hourglass in Stage 2, and use it to freeze the boss when he swoops down towards you, which leaves him completely vulnerable to attacks for four seconds at a time. You can also grab a Shield B (the reflect shield) two screens before the boss, which allows you to block the Bat's fireballs. The Morningstar whip is easily the best primary weapon for this fight, given how fast it can hit enemies.

Medusa
Medusa is by far the easiest boss in Vampire Killer, and only becomes a threat if you let the battle continue for more than 10 seconds. If that happens, there's a health orb in the leftmost candle in the boss room.

The most effective item combo to use in this fight is the Hourglass with Holy Water, which allows you to freeze Medusa in place and then shred her HP to zero in a matter of moments. VK's damage-per-hit to bosses cap, conveniently, does not apply to sub-weapons. Alternatively, you can freeze Medusa with the Hourglass and then attack her with whichever primary weapon you have on hand, though this method will require you to use the Hourglass more than once for it to be successful.

The Mummies
The Mummies are a very easy boss, as long as you have the proper items in your inventory.

Entering the fight with Shield B in your inventory and holding crouch upon entering the room will allow you to attack the boss safely - you're immune to the Mummies' bandages due to the properties of Shield B, and outrange the Mummies' melee hitboxes. The Throwing Knives are the best primary weapon for this fight, given their screen-wide range.

Frankenstein's Monster
Frank is the first truly dangerous boss in Vampire Killer. This fight can get dire very quickly without the right equipment. Frank's back and forth movements are quick and erratic, and he deals massive damage to Simon on contact. His Fleaman companion definitely doesn't help your situation either.

Holy Water is extremely useful for fighting Frank, as throwing a jar of the stuff at the monster's feet will deal out heavy damage as Frank runs over the flames. With perfect aim, you can two-shot him in this manner. Note that attacking the Fleaman will only stun it for a couple seconds, and will not decrease the boss' total health. If you can, try to take damage to either the Fleaman or its fireballs, as they deal far less damage than being hit by Frank himself.

Death
Death is arguably the most challenging of Vampire Killer's bosses during a standard playthrough. Death's movement is a lot faster here than in NES Castlevania, but he's also far more predictable given his simplistic flight patterns. His mini scythes also don't deal immediate damage when spawning, which gives you a few extra frames to get out of the way.

The best plan for Death is to pick up an Hourglass, and use it to freeze him when he's level with the boss room's central platform. You'll be able to get in a bunch of hits with whatever primary weapon you're using during those four seconds. Repeat this move until Death's health is depleted to zero. Also, grabbing a Shield A (defense shield) will half the contact damage you take from Death himself, which can be very beneficial for tanking extra damage.

Count Dracula
If you've got the right items, enough hearts, and your wits about you, the final fight against Dracula won't pose much of a threat.

Phase 1 is your average initial Dracula battle: teleportation and fireballs. Dracula's only hitbox is his head, so you'll have to jump to get high enough to deal damage to him. The Throwing Knives are an excellent choice for this fight, as their screen-wide range allows you to attack Dracula no matter where he appears after teleporting. To further decrease the difficulty of this fight, the Hourglass allows you to freeze Dracula onscreen, making it easy to get in several extra hits on him before he teleports again. You can even one-cycle Dracula's first phase consistently if you chain Hourglass uses quickly enough.

In Phase 2, if you grabbed the Shield A earlier in Block 6, the bats summoned from Dracula's portrait shouldn't cause you any problems. Climb up the blocks on either side of the boss room, and use your primary weapon to attack the glowing oval on portrait Dracula's forehead - this is his only damageable hitbox. Once again, if you have the Throwing Knives, this fight will be easy, and you can use any Hearts you still have to repeatedly freeze the bats with the Hourglass.