Out of Character

these guys only have 12 AC, so even with disadvantage most of you have +10 or +11 to hit so only a nat 1 or 2 will miss
They are mostly tough because they have 66 hit points, so it may take 2 of you ganging up on 1 to kill it, rather than each killing one each round
Their saves aren't great, max +3, but they have magic resistance

i dunno if healing me is the best choice right now. I have advatage of death saves so i have a decent shot of popping up on death saves.

Probably wait a turn or two unless we get to the point where i can actually it him. I could also be a meat shield if healed though.

Context: When I begun playing 5e I found a lot of rules inappropriate, inadequate or simply incomplete. I was inventing a lot of homebrew to "fix" 5e. It always ended up either with players not having fun or finding ways to break my invented rules either making their PCs OP or just breaking the challenges I was putting them into.

Lesson learned: After years I've realized that most of things I perceived as a flaw in 5e are just a workaround to make game balanced and fun for players. Everything is very carefully worded to avoid situation abuse. There are very few flaws still left in the game that they are trying to fix in One D&D, but these are really minor balance fixes.

Example: Jeremy Crawford says
But as DM, you may change how anything works—and deal with the results.

I'm not as educated as him on the results, but I could provide one example for instance - if DM change dragon breath weapon to deal damage to all non magical objects in the area, not just creatures... the result should be (shouldn't it) - anything PC is wearing - including their backpack, spell ingredients rations and what not - be destroyed from a single enemy attack.
As a player I would rage quit a game where that happens.

I get where you are coming from. I think there is a small conflation between "can also target objects" and "will destroy your backpack along with hurting you". It is pretty common in the game for objects to only be harmed is they are not being worn or carried. I never meant to imply that because a might dragon's breath weapon should be able to damage a stone wall that if you got hit by the breath weapon you might suddenly be naked from everything you own turning to dust. When a player is in the cone of a red dragon breath weapon, I don't burn any of the objects they carry, but if they were hiding behind a tree that is now in the area I think it is reasonable for the tree to be harmed or at least catch fire. No one is holding or wearing the tree.

It is great we agree carried items become immune.
Our only disagreement is weather the part "A nonmagical object that isn't being worn or carried also takes the damage if it's in the spell's area." of the spell Shatter makes this spell unique or a DM rule every other spell/feature that does area damage has the same ability.

I believe everyone have the right to have their own opinion weather - quoting Jeremy they play in reality where "Some spells are drawn to or harm only the life force of creatures."
Here is my opinion: if I compare the damage a flamethrower or a liquid nitrogen could do to a human body VS a stone wall. Compare throwing a wooden plank on high voltage wires VS throwing a human body.... I fail to see similarity in damage taken. Even A tree will burn for hours if set aflame - one round (6 seconds) of a combat will reduce max HP by 0.1%).

Even with RAW you should be able to able to apply reasonable expectations to a particular scenario. For instance, if you’re holding a piece of parchment in your hand and get hit with fireball… Its reasonable to say that parchment is gone, Or if a black dragon spews acid all over the place… its reasonable to think the ground is now contaminated to some extent.

The point being RAW is fine for broad rulings but to rely on it exclusively without regard to reasonable expectations turns a what should be a colorful fantasy world black and white.

I believe we are questioning our expectations. We are not (jet) questioning weather rules are in sync with our expectations.
The question I am trying to define my own expectations about is this: are all spells and features the same and just doing damage? Are they all unique?
Let's compare Shatter and chromatic orb mentioned previously.

one spell "dealt some degree of injury to its target".
another one chatter "produced a tintinnabulation of sound so powerful that it could damage or shatter brittle objects".

Should it be the expectation of Shatter to destroy 20 feet of wooden fence, while chromatic orb's "injury" only apply a visual effect on the wall. Or should chromatic orb affect fence structure destroying a 5 feet of it?

Medusa Petrifying Gaze is very specifically described explaining that you don't need to close your ayes - you just can't look directly at the target creature, you can still look at any other creature.
Medusa gets advantage when attacking as it counts unseen and PC gets disadvantage attacking medusa being unseen, but any of Medusa's allies don't benefit of that.

Optional Rule: Facing
If you want the precision of knowing which way a creature is facing, consider using this optional rule.

Whenever a creature ends its move, it can change its facing. Each creature has a front arc (the direction it faces), left and right side arcs, and a rear arc. A creature can also change its facing as a reaction when any other creature moves.

A creature can normally target only creatures in its front or side arcs. It can’t see into its rear arc. This means an attacker in the creature’s rear arc makes attack rolls against it with advantage.

Shields apply their bonus to AC only against attacks from the front arc or the same side arc as the shield. For example, a fighter with a shield on the left arm can use it only against attacks from the front and left arcs.

Feel free to determine that not all creatures have every type of arc. For example, an amorphous ochre jelly could treat all of its arcs as front ones, while a hydra might have three front arcs and one rear one.

On squares, you pick one side of a creature’s space as the direction it is facing. Draw a diagonal line outward from each corner of this side to determine the squares in its front arc. The opposite side of the space determines its rear arc in the same way. The remaining spaces to either side of the creature form its side arcs.

On hexes, determining the front, rear, and side arcs requires more judgment. Pick one side of the creature’s space and create a wedge shape expanding out from there for the front arc, and another on the opposite side of the creature for the rear arc. The remaining spaces to either side of the creature are its side arcs.

A square or hex might be in more than one arc, depending on how you draw the lines from a creature’s space. If more than half of a square or hex lies in one arc, it is in that arc. If it is split exactly down the middle, use this rule: if half of it lies in the front arc, it’s in that arc. If half of it is in a side arc and the rear arc, it’s in the side arc.

A petrified creature is transformed, along with any nonmagical object it is wearing or carrying, into a solid inanimate substance (usually stone). Its weight increases by a factor of ten, and it ceases aging.
The creature is incapacitated (see the condition), can't move or speak, and is unaware of its surroundings.
Attack rolls against the creature have advantage.
The creature automatically fails Strength and Dexterity saving throws.
The creature has resistance to all damage.
The creature is immune to poison and disease, although a poison or disease already in its system is suspended, not neutralized.

Hythonia has 209 hp and AC 20
She drank multiple potions, effects of which I will reveal only if asked for. You can guess at least 1 of them.

Figured she powered up, I would have given her invulnrability and the least, maybe flight and giant strength

Invulnerability is tempting, but I prefer fights that go fast to ones that are drawn out, so giving the boss effectively double hp with no counterplay felt less desirable than other things.

You are right about 2 of the 3, though, and a close look at the rolls may hint at the 3rd

Great white sharks can generally swim in water as shallow as 1-2 meters (3-6 feet) in depth. They are capable of temporary excursions into even shallower water, such as 0.5 meters (1.5 feet)"

I work in IT. I'm engineer. When I play with my IT colleagues we assume rules exist to limit both the play and the DM. We don't allow DM to limit PC just as we don't allow enable PC to do what he is not supposed to. The engineer in me scrams a medium sized creature could squeeze into 2.5x2.5x2.5, not less. You possibly fit your PC into 1 foot thick water.
But here on GM we are not all engineers. I'm learning to be flexible in my view on 5e...

The question that I would ask - as a DM what do you think a magic does in your world? When one cast wall of water - does it summon a real physical water? 30 feet long, 10 feet high, and 1 foot thick. It is is 300 Cubic Ft. = 18723 Lbs of water. Should a DM calculate if floor where the magic is cast is able to maintain this much weight? Would a wooden bridge collapse when you cast this spell on that bridge? Does archimedes' principle work in that water? Do we want to apply all laws of physics apply our adult knowledge of anatomy, biology, chemistry when we decide effect of the spells to the natural world? What if there is a 12 years old player at your table?

While you are wielding a glaive, halberd, pike, or quarterstaff, other creatures provoke an opportunity attack from you when they enter the reach you have with that weapon.

Move. Hythonia moves up to her speed without provoking opportunity attacks.