Boss Rush/Enemy Gauntlet

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So me and my Friends recently startet playing DnD 5e.
And most of us are not quite used to the fighting style. And to train that I got the idea to make a Boss Rush/ Enemy Gauntlet Mode for them.
This Mode would be more focused on combat and fighting styles, so the players can improve on that instead of just saying "I stab him with my sword in the butt".
I would balance the mode around level 10. with the level being locked in.

The Mode would also get harder as the players defeat more and more enemies, these would have a tier ranking going from tier 4 to tier 0 where tier 4 stands for a beginners boss and tier 0 for something unfair / engame material.

For now I am asking you just for ideas for boss battles I could introduce in this case everything goes.
If you have any ideas for a boss post them here with a picture maybe some stats and attacks + the tier where you think the boss belongs!
I am thankfull for everyone that posts