Name: Magma Serpent
HP: 9 (I assume, though the amount dedicated to a mini-boss might be different.)
Location: Mini-boss or Boss of the Carbon Mines (There might be some later, which you can use the Bomb Cannon on as well).
Description: The Magma Serpent is a creature that lives below the earth, generally in the pools of magma in and around Death Mountain. They are extremely territorial, and will exterminate encroaching threats with extreme prejudice. Their bodies are similar to a Magtail's (WW), but they bear resemblance to a snake or dragon, with fangs and the capacity to spew "venom", in this case, lava. They hate the cold, and if their bodies become to chilled, by wind or water, or simply prolonged time out of lava, they will withdraw before attacking again.
The Fight: The fight with the Magma Serpent takes place based on the amount of heat left in it's body. When it's fully charged, as in when it just surfaces, it will be all bright and glowing cherry red in its lava veins. However, when it attacks with Lava Spit, when you hit it with an Air or Water Cog attack, etc, it will lose some of that heat. Once it runs out of heat, it will turn gray, leaving it vulnerable to an attack. Once hit the appropriate amount of times, it will use Burrow, or retreat to an already existing hole if two already exist. Once it's underground, it will refresh its heat.
Attacks:
Eruption - 1 HP
The most basic, and easily avoidable of the Magma Serpent's attacks, is the Eruption. Basically, the attack consists of a small explosion of rocks as the creature surfaces, burrowing its way out of the ground at speed. To avoid this attack, simply watch for the preceding animation, and stay out of the way. (Note, it will use this attack a total of two times, leaving two exit holes, after that it will come out of an existing one.)
Burrow - 1 HP
Similar to the Eruption, the Magma Serpent, after receiving damage or when its body cools, will dive back under the ground, leaving another hole in its wake. This one is slightly more difficult to dodge, as the Magma Serpent gives very little indication of when it will dive, aside from its body's cooling or damage received. To avoid it, you simply have to retreat from the creature once you injure it. It will use this attack once it has run out of heat.
Lava Spit - 2 HP and Burn
As the Magma Serpent chases you around the room, it will occasionally shoot flaming balls of phlegm at you, which will set a little patch of fire on the ground, and deal damage if it hits you. This attack causes the Magma Serpent to lose heat, and it can only use this attack three times before it will retreat underground.
Player Countermeasures: While fully heated, the Magma Serpent cannot take damage from anything. The goal is to lower it's temperature by either prolonging it's time above the ground, or hitting it with cooling attacks.
Sword - Hitting it with your sword will bounce it back somewhat, like attacking a Darknut's shield in Minish Cap, thus useful for barring it from entering it's holes. Once it runs out of heat, the sword can be used to deal damage to it until it burrows again. This contact removes 1 heat charge.
Shield - Blocking / Bashing it with your shield will bounce it back somewhat, like attacking a Darknut's shield in Minish Cap, thus being useful for barring it from entering it's holes. This contact removes 1 heat charge.
Hookshot - Has no effect
Bow and Arrows - Shooting the Magma Serpent will bounce it back somewhat, like attacking a Darknut's shield in Minish Cap, thus being useful for barring it from entering it's holes. This contact removes 1 heat charge. Once it runs out of heat, this attack can be used to damage the Magma Serpent.
Forest Cog - the tornado created by the Forest Cog is incredibly useful against the Magma Serpent, as it removes 4/8 heat charges. It has no effect once it has no heat left, however, except to put out the Lava Spit patches.
Water Cog - the ring of water created by the Water Cog is incredibly useful against the Magma Serpent, as it removes 4/8 heat charges. It does its appropriate amount of damage once the Magma Serpent has no heat left. It can put out the Lava Spit patches.
A Word from our Sponsors: To clarify a few things:
The Magma Serpent has 8 heat charges. Hitting it with stuff removes a charge, hitting it with a cog removes lots.
Once the Magma Serpent has no heat left, it gets stunned for a few seconds. During this time, you can damage it.
After the stun wears off, it will Burrow again, leaving another potential surfacing point (a la Whack-a-Mole).
I wrote this with the idea of it being the mini-boss, but I realized that it would work just as well as a Boss if a few things were tweaked.
Since you would have the Bomb Cannon, you would be able to injure it like that, by either getting a lucky hit above ground, or dropping it down the right hole as it goes to surface.
Or, if you're planning on using Bomb Flowers, have them usable the same way ^^
Its heat and its health amounts would increase, and it would need another attack, probably a bigger Lava Spit, for when it enraged.
There could even be two of them!
What do you think?