I see where you are coming from, but all I see is a designer vs player mentality.
Now, to go to your fire vs water example.
Is this telegraphed?
In that battle? No. In the game? Yes. However, it relies on the player doing things the player should be doing.
Namely, talking to NPC's. There is one fairly early on that mentions the "Revenge" mechanic. Let me see if I can find the text for you.
"Strong monsters don't like it when you exploit their weaknesses and will hit you hard when you try to do so."
From that point, the player will see it in action at some point, and note that "all bosses have this mechanic".
The mechanic itself isn't meant to cause a "Game Over". It's meant to hinder you for a few turns and teach you "don't do that" or "be prepared if you do it".
The initial enemies you'll run across that do this are either a Plant or a Spider (story choice determines this boss). The Plant does "Foul Bile". It hits everyone and only inflicts Level 1 and Level 2 Poison on them. No damage. It does this if you hit it with Fire. The spider does "Maul". Hits one character for a good amount of damage and inflicts Level 1 and Level 2 Poison on them. Only happens if hit with Fire.
These are "safe bosses" to help ease the player into the mechanic.
"Player Prep" is something that is encouraged across all elements of my game. If you go in unprepared, you'll die. That simple.
As a dev, I respect the player and don't treat them like a brainless child who needs coddled. I signpost everything as I can and then pull the Training Wheels off and expect the player to figure things out. I expect them to get hit with stuff and then figure it out.
Did the players encounter a weaker version of the fire elemental that showed them this was a bad idea?
Or did you just spring it on them, just for this boss, with Water working fine on all other fire elementals?
Water works fine on all Fire elementals, yes. Fire works fine on all bugs and plants too. It's just the bosses that are different.
Players are warned the mechanic exists... will very likely accidently trigger it on one of the early bosses, see the effect, and make a note of how the mechanic works. When hit with the Revenge attack, players are even told, "X seeks Revenge!".
Another adage to your 'some bosses only punish turn one use of limit breaks but not turn two usage.' How am I as a player supposed to know this? I feel like a lot of times in your games, I would be making decisions blind, not getting it right, and just reloading if I get it wrong.
Players who go into my game expecting me to tell them exactly how to win the fight on their first encounter of it are going to have a bad time. Might just not be a game for you. If you feel the need to reload just because your Turn 1 Limit Break was wasted... Well, that says more about you than it says about me. Likewise, if you could've only won because you blew your Limit Breaks on the first turn... you're not a very good player and haven't learned anything in the game.
The player is meant to experiment, explore, figure things out. They get hints along the way and nothing more.
Skilled players who have learned, have experimented, have explored... will have a decently easy time with the game. Those that didn't do any of these things... gonna have a bad time.
But, if you're looking for a "strategy" involving these... All you'd really have to do is use one Limit Break and if it isn't healed off in 1-3 turns, you blow the rest of them. The way the mechanic works is "if you've drained a specific threshold of health by X turn, use Full Heal on boss". If you don't reach the threshold, even despite using the Limit Breaks... still safe. Limit Break isn't wasted. It's up to the player to determine where and when and why they use their resources.
Players are absolutely thrown curve balls now and again (or as often as I can, really). Strategy isn't about knowing what's coming and so it doesn't affect you. Strategy is figuring out the counter to something that hits you so it doesn't affect you AGAIN.
I'm not making some "easy mode" game where I advertise all the mechanics up front before every fight so that you'll beat everything on your first run. You're going to see the Game Over screen if you keep failing counter things. Characters are designed to die in about 4 to 5 hits. You've got wiggle room to figure things out. If you can't figure it out by the time you've been hit 5 times... that's a "you" problem.
If you're making an argument for Limit Breaks to trivialize boss encounters... I guess I'm not sure why you'd want that except to "easy mode" the game and "destroy mechanics".
I mean... hell... I did that crap in Borderlands 3. Shotgun spec where every pellet had a chance to inflict a Critical... on Critical hit, there was a chance I'd get my clip automatically refilled. Shotgun with 15 pellets, crit chance of about 50%, never needed to reload... fire endlessly... but my rounds weren't shotgun pellets... they were remote mines. Pile all the remote mines onto every boss possible... then detonate them all at once... end the boss instantly and NEVER see it's "phase" mechanics. Because... seriously... screw your phase mechanics. If I can get an easy win, I'm going to do that.
Additional point; Yes if the adds die in one AoE, then they were useless. That's why you make the adds take multiple hits to kill. You also account for the adds in the boss HP allotment. One 10.000 HP boss, or an 8.000 HP boss with two 2.000 HP adds for example.
So then you're really just increasing the damage output of the boss instead? Instead of the boss hitting you once, it's effectively hitting you 3 times? You focus down the adds to reduce amount of incoming damage and then attack the boss?
I guess I fail to see why "Adds" is anything useful in a boss fight unless they add strategy. You could effectively just give the boss 10,000 HP and just make it hit 3x as hard for a few turns for the same effect. Or, just AOE to simulate "spreading damage around".
If the player can expend a few hits to wipe out the adds in the next turn or two, they don't really add much, especially if they're not doing a lot of damage, can be neutralized with states, or any other number of methods. Likewise, adds tend to be a "minimal threat", so nobody sane is going to blow a Limit Break on the adds when they can slaughter the boss with it and mop up the adds later.
Now, if your adds were HEALING the boss... or SHIELDING the boss... you've got something there. But, that's sort of the same thing as "Hit Sponge". It might be executed poorly enough to annoy players, or could be done in an interesting way so players have to employ strategies to destroy it.
Now, to answer the more HP vs More strategy point, I believe a boss needs some HP for strategy to unfold. A battle over in five rounds didn't require much strategy.
It does and it doesn't. You simply need to employ measures to make combat take longer. Bosses can employ "stalling tactics" as an example. If your states are properly dangerous, you could full poison an entire party and force the player to remove the state before beginning the attack again. If curing of this state isn't AOE and can only be done 1 at a time, then you must spend actions removing it before resuming the attack.
No need for high HP if you have stalling tactics. The reason most people employ "Hit Sponges", is because they don't really know how to employ that "lateral thinking". They just see, "If you have to hit it more times, you get to see the mechanics" and nothing else. The end goal is "The player needs to see all the mechanics". The main way of getting there is "it needs to last enough rounds". How you get enough rounds can be done in several ways and not just "hit the boss more times".
A boss needs time to teach you its mechanics, test you on them, and ultimately try to kill you with them.
I always assume a player reaches a boss at full power. I want him to not die in one ultimate attack, but a spiral of death as he makes wrong decision after wrong decision.
I don't disagree. Do you think my "Revenge Mechanics" are boss designs are "kill you in one hit"? They should only kill a player who extraordinarily bad at the game. Namely, they keep making mistakes. They don't do what they're supposed to. Revenge kills you if you are already on the ropes. Otherwise, the most it might do is "stall you". Force you to spend some actions healing up, or some actions removing states. It kills you if you're mismanaging your play.
The boss needing time to "teach you it's mechanics" is why I heavily regulate the amount of power my players have and why some bosses will outright remove damage you've done with a Limit Break. What sense does it make if I let you build up the Limit Break on weak battles outside the boss and then allow you to use that Limit Break to effectively destroy the boss? What sense does it make to design every single boss in the game around the player doing EXACTLY that and then have to buff their HP in order to prevent "turn 1 kills"? This is really weird thinking and I don't know why devs engage in it. Devs creating a problem and then creating a solution to that problem. Just remove the problem!
The 20% weaker equipment is also how I handle it. If players want to "steamroll" the boss, they're going to be eating more damage OVERALL from ALL SOURCES for that privilege. They're going to be doing less damage OVERALL as a result of that choice.
That's on top of the odd boss here or there that will simply heal away your Limit Breaks if you blow them on the first couple turns of combat. Turn 1 Limit Break Dumps are viable for most of the game... except on those bosses where it isn't. When it isn't... you now how to THINK about how to beat the boss. Oh noes... I'm punishing a player by saying, "You actually need to think in this fight, and it's going to hurt you because you've been doing your best to NEVER THINK during the entire game!".
That's my two cents on it, anyway. I don't want any one strategy to work 100% of the time. Because it detracts from the behavior I want from my players. If I teach you that X strategy works... I'm going to have bosses where it DOESN'T work and in fact uses that strategy AGAINST YOU. Keep you on your toes. Keep you from getting complacent.
As for teaching...
I don't approach boss design that way. The monsters you fight BEFORE the boss should teach you the mechanics you need to know to beat the boss. Employing what you've learned before should grant you baseline survivability in the boss fight. The boss should then test you on what you've learned and then employ one or two NEW mechanics on top of what you've learned so that it is a proper test.
Baseline monsters are your Teachers, Bosses are the Tests at the end. I don't design bosses where all their mechanics are brand new so you need to see them all. You need to see the two new mechanics and nothing more. The rest of the mechanics are just a test on stuff you SHOULD HAVE already learned by getting this far. So, if you die... it's because you didn't learn the mechanics the baseline monsters were teaching you.
Now, that being said, a boss should die once the player has shown he 'got' the boss. No need to make him repeat the same routine ten times.
And one final point, the boss healing is turning him into a HP sponge. A boss with 12.000 HP and a 10.000 HP one that heals for 2.000 are the same.
I agree. My biggest issue with Hollow Knight and games like it is this very thing. "I figured out the boss pattern 12 hits ago... why do I still hafta land 70 more hits to win?" The danger is just in falling asleep at the wheel, rather than it being any challenge. Makes the gameplay very... BORING. Makes bosses FRUSTRATING.
I have the same issue with Raids in MMOs. If I can demonstrate my proficiency in this fight up to a specific degree, why am I not granted all the loot it has to offer? Why must I keep coming back every single week like a robot to get it? No. I've beat it once, without taking an avoidable hit... that should be enough to grant me all the loot at once.