Bartender is pretty simply. You simply need to attack people using a bottle. The only downside is that once a bottle is used, it breaks and you need a new one. However, if you play the level "Welcome to Hope" then you'll be in a bar with plenty of bottles and plenty of people to fight. Just keep grabbing the bottles that line every counter and shelf. Once you have one, approach anybody and hit the X/Square button to hit them with it. This will finish the fight in a single hit.
Do this a few times and you'll have this style.
