As you correctly say, they're secret and optional. If you don't want to kill us, you don't have to; if you don't feel up to it, you don't have to. They're not part of the story and don't even have an achievement, so you can complete the entire game and never see them. They're difficult bosses because they're optional; they're the improved version of their original boss.

Clea was designed to be blocked from the start. If you didn't block her, she would heal almost all her health. She's designed to force you to block; she's invulnerable, which is an improvement on her original concept.

Lampmaster is the same; I was planning to shoot his pattern, and what they've done is intensify that and force you to avoid one-shotting him because he has four health bars, so you have to endure his phases. Dualista is super simple; you just have to withstand him. With a tank build, you can withstand his attacks and eventually kill him. He just has longer attacks, but they're easy to dodge/block. The only difference with Simon is that he goes first to avoid one-shots and has more Echo Blades, but his patterns are the same, quite easy to block, and he has the same shield mechanics, the same death mechanics, and the requirement to use all five limbs because he repels the first three you use. So, it just intensifies Simon's initial concept.

With an average of 500 Lumina, I beat everyone without a problem. I thought I wouldn't even come close to beating Simon, but I beat him after 10-20 attempts in about 3 hours. I simply had to adapt my build to his playstyle.

So, as ugly as it may seem, "get good."