As you've heard already, this is a anime stereotype where a character is violent and defensive at first, but sweet and affectionate in later on.
If you get a tsundere mad, you're best bet is running.
A doujinshi is a version of a manga, but made by amateur artists. They are usually cheap, though are really good quality. Doujinshis aren't exactly famous unlike the other mangas out there.
Person #1: I loved this doujinshi!
Person #2: Eh, the other doujinshi from earlier was better.
This is basically a Japanese internet slang used commonly to define people who are obsessed with the Japanese culture.
Some people see them as annoying, since they always like to imitate the Japanese culture, even if they're not Japanese.
Person #1: Arigato for the food! Kenpai!
Person #2: Since when you've become such a weaboo?