The PDW-57 is a low damage high fire rate sub machine gun which is the second machine gun you unlock. It has medium recoil (similar to the MP7, click for a guide for the MP7). It is best suited for an aggressive flanker that goes after the objective in those modes that have them. The PDW-57 has reasonably good hip fire characteristics, but your are going to do much better if you aim down the sights.
At the end of this guide is a YouTube video with a visual version of this guide.
The PDW-57 has trouble when you oppose more than one person at once. The high fire rate lets you effectively deal with one enemy, but any more than that and the time it takes to kill will hinder your abilities. It is best to flank the other team and encounter them one by one when they are unaware. Stay on the edges of the maps and approach the current objective from the sides or behind.
When you do run into an enemy that is shooting back start hip firing but at the same time aim down the sights so that you are firing whilst aiming. At the same time start strafing left and right because, unlike an Assault Rifle like the MTAR (click for a guide for that weapon) you move quickly whilst aiming down the sights which helps avoid enemy fire.
The class setup I recommend is the following:
Lightweight helps you flank faster as it increases your running speed and Dexterity helps you when you are caught unaware as it helps you pull up your weapon faster after sprinting. When you are firing and aimed down your sights Toughness reduces flinch when you are shot which makes killing enemies that are shooting back easier.
For your lethal grenade I recommend you use the Semtex Grenade since it is fast to pull out and explodes soon after you threw it supporting a faster combat. The tactical grenade you choose is up to your preferences, but I recommend you use concussion or flash grenades. A pistol secondary is an excellent tool when your primary is empty in the middle of a gun fight.
When you are aiming down the sights and strafing the faster you go the better you can avoid enemy fire. The Adjustable Stock increases your movement speed when you are aiming down the sights. When you are flanking sometimes you run into a series of enemies, or even maybe 2 at once, and your gun might not hold enough bullets to deal with all of them. Extended Clip increases the number of bullets per magazine.