Beating bosses in Dark Souls II can be frustrating if you're a beginner, and playing against others is no different. Juutas on YouTube has a surprisingly well-edited clip, mostly for beginners, showing and explaining 12 techniques that are often used during PvP invasions.
Here's the full clip. Don't let the Xbox 360 controller confuse you, these tricks work with every version of the game.
Or, as an alternative, you can just dress up as an enemy and mess up everything.