1. Stay with your group.
2. Revivie other payers as soon as possible. You can ignore small groups of enemies.
3. Hold block while reviving other players or looking around.
4. Don't shoot the dwarf (or any other teammate). If someone is holding a chokepoint at melee range, do not fire in his back. Use your melee weapon and together you can stagger and kill hordes easily.
5. Use the environment to your advantage. Fighting hordes at chokepoints can be a lot easier than fighting in the open field, surrounded from all sides. Stacking cleave and stagger, it is easy to mow down huge hordes.Don't fight bosses in narrow areas, if you don't have to.
6. Every pickup throughout the mission is a resource for the whole group. Good players will use less healing and share ammunition.
7. Blocking/ Dodging. Learn to avoid damage and which attacks can be dodged or blocked.
8. Pushing enemies will draw their attention. You can also push to help teammates or reposition yourself. Pushing hordes instead of spamming light attacks helps with holding your ground instead of falling back constantly.
9. Tag everything. Your teammates can only kill the Stormcaller, if they know where it is.