In episode 4, Nagasumi is challenged to a fight by two bullies twice his size. Knowing he can't beat them in a straight fight, Nagasumi grabs them both and jumps into a river, where Sun is able to discretely take them both out.
The massive fight between Sun and Lunar's dads in episode 10.
For Sun's awesome moment, we have episode 13.
The end of My Bride Is a Mermaid features a massive brawl involving mermaid yakuza, songs of mass destruction, gigantic eels, and the Terminator himself in a seifuku firing laser beamsout of his eyes while flexing while stuff continues to explode in the background. Meanwhile, the badass normal male lead gains the ability to catch bullets in his teeth and punch people through walls out of sheer love and righteous anger, and permanently Art Shifts into a muscle bound icon of exaggerated manliness by the epilogue. Even following the entire series can't prepare you for this stuff.