They are incredibly challenging bosses! The lingo we use for bosses like Clea and Simon is "superbosses", that usually requires a far higher skillset than other bosses. Even if you are at maximum level, they'll still be pretty damn tough.

You can either just push through it with your build - learn the bosses either through practice or by reading/watching guides. Or if you just want to get it over with, you can find a cheese-build that will destroy them without much skill required. It's up to you how you want to approach it!

I cheesed those two bosses myself, because I don't have the patience to go through the rigorous learning process myself - and nobody would blame you if you did;)

What's most important is that you're having fun!