Let's be real here for a moment.
There's nothing unique or interesting about those fights that would set them apart from any regular story boss encounters, and the only challenge stems from the hideously long health pools of those enemies, and their uncanny ability to drain your own health bar in three hits. So, in other words, the only thing that's in any way challenging about those fights is trying to stay awake throughout the whole thing because each of them is so mindlessly repetitive, and they go on for such a goddamend long time that I literally start thinking about something else while I'm mashing the attack button and dodging the same repetitive attacks, until I inevitably get distracted enough to slip up and let the enemy land a hit on me, essentially ending the fight because they used a bullshit special attack that ended me in one hit.
I swear, those are some of the worst boss fights that I've seen in a while in an otherwise excellent game.
My main issue is most of those fights are not one on one. It is extremely frustrating when you are trying to focus on main boss and get distracted by minions. You have a very limited field of view on tv screen and it’s just impossible to have more then one opponent and be fair to the player. Dying in few hits is fine as long as I get out something from the fight which usually is better understanding the boss. Adding minions just interrupt the whole process. Most of them are off screen you have no idea what they are doing and you are just reacting to yellow/red lights, might as well play dance dance Revolution then.
Soulsborne nailed this but there are f**kups around this in Elden Ring, hopefully they will be careful in future.
I'd say you basically have to git gud and level up or learn the game, and also know that some optional bosses are designed for the end game, once you completed the game and already leveled up enough and unlocked a lot of stuff.
When there are several enemies you can position yourself strategically in a way that see most enemies and have close to you only a fair amount of them instead of being in the middle of the battle surrounded by enemies or always just in front of the boss. When there are minions typically it's better to go away from the boss and without stopping to giving him attention due to long range attacks kill the minions while being far also to have time to collect the health orbs they'll leave when dying.
In addition to this, the game also provides growd management or area based related attack and skills. Plus also a roll evade movement that avoids basically all attacks that could come from behind. Plus certain enemies are also weaker to certain elemental attacks or type of support attacks specially. Try to learn what is more effective against each one instead of mashing the same attack button and the dodge button.
Same goes regarding using dodge as defensive option, remember there's also the shield parry that is the way to go in certain types of attack.
If you die in a few hits it's because you don't have equipped the proper build to defend against these kind of attacks, or that there is a quick way to kill that enemy using certain attacks that you may have not still unlocked or that still didn't upgrade your health bar.
Meaning that this optional boss pretty likely requires a higher level than you have so it's better to leave it for later or try to make a more appropiated build of weapons, special attacks, armors, type of support attacks etc. In general it's a good idea to complete first the main story and leave the optional stuff -particularly the optional bosses- for the end game once you completed the game.
Regarding the waves of minions, they are a temporal challenge inside the combat that as reward gives you extra health by killing them. In terms of game design it justifies a way to give you extra health that will be welcomed in a boss fight but require some effort from the player to earn it, in addition to make the combat more dynamic and less repetitive adding a different pattern to the ones the combat already has.
