you have to learn how to face the bosses, that's all
you can get through the game with uneffective equipment and stats until you get to the
fake last boss, where if you're not prepared right, you pretty much don't stand a chance unless you coop
and if you get through it by cooping, you'll have a hell of a hard time in your first newgame+
also, the stat symbols are never explained, you have to check them up and memorize what they mean ;/
EDIT: Useful tip for people, aswell
When you figure out how this game works, with players basically giving tips to each other as messages or just popping up to momentarily aid you, if you think outside the game, a wiki is the closest fitting thing for this mechanism
Just so happens there are two wikis (watch out for spoilers, but you can get through them without seeing any)
http://demonssouls.wikidot.com/ Very good wiki with few spoilers
http://www.atlus.com/mediawiki/index.php/Main_Page Somewhat good wiki owned by Atlus, however the user content is of somewhat lower quality than the other wiki, and it's easier to find spoilers here; Some walkthrough info might be more in-depth though