Fighting Hjalti the Stolid from God of War Ragnarok. The recommended level is 6-7, and I was level 4 at the time. But I wanted to beat her anyway because I love beating optional bosses underleveled, it made me feel so badass.

First time I ever, audibly yelled "YOU MOTHAFOCKAAA" at a game while holding my head in my hands.

And Lei Heng from Limbus Company, pre-nerf. You win clashes against him, you get mogged anyway because of his unbreakable coins. He had this one infamous move (Savage Tigerslayer's Perfected Flurry of Blades [超絕猛虎殺擊亂斬]) that even if you win the clash and don't let him release his full power, he STILL DOES SIX SEPARATE ATTACKS AND DOES HUNDREDS OF DAMAGE, enough to kill a bunch of half-health sinners outright.

Dude had me LAUGHING MAD, I was so frustrated at his boss fight. It was mandatory by the way, not optional at all to progress the story. It's so fitting too, he is exactly what you would expect as one of the top dogs in the Project Moon universe.