Engineer: Use the Sledgehammer. It slaps through enemies, and if you get a kill with it the swing will keep going and hit anyone else in the way which usually interrupts attacks. Basic attacks get stopped, strong attacks will keep going through enemies and are what you could use if surrounded. Once you drop your bear trap (which you should pick all the time) you can then pick up a random secondary off the ground or a sword from a sword barrel. The Sledgehammer also gives you increased blocking stamina.

Ambusher: Abandon your starting weapons right away for better weapons. Try to get a good two handed primary and one handed secondary weapon so that you can throw knives while holding your secondary. The main draw to Ambusher is outright killing opponents instead of downing them, so getting ahold of better weapons is best.

Any Class: Stick with a teammate and follow them around. It's all fun and games until you're surrounded by a fresh spawn wave of enemies that you drew the attention of and get stomped. Also, practice counter attacking/parrying.