Switch Characters
Although the AI controls the Characters you are not using you still need to make sure you keep yourself alive. Switching characters when the current one is low on health will help you accomplish this. The squad will follow you and triangulate based on your position.
Icons
Pay attention to the icons that appear on screen as they indicate enemies are nearby but not in your direct line of sight. If an enemy is approaching an exclamation mark will appear, the bigger the icon bubble the closer the enemy is to you. If the exclamation mark turns red it means the enemy is about to attack you.
Special Move
Each of the Characters has their own special move so look to take advantage of it whenever possible.
Surrounded
Always position yourself to prevent being surrounded, keep an eye on enemies as they will try and come around edges and sides of areas you think are protected.
Supply Drops
You will find a Supply Drop after every two or three waves of enemy attacks and they will offer you several upgrade choices from which you can select one. Don't choose the option of refilling the armour of your current character unless it is dangerously low on health, it is more beneficial to pick the passive upgrades like the higher health drop chance or increased damage.
Recharge El Diablo
Make sure you use the short break between the waves of enemy attacks to recharge El Diablo or load up on nearby ammo.
Friendly Fire
Teammates cannot be hurt by friendly fire so don't be afraid to attack an enemy when one is nearby.
Pausing
Althrough you cannot save your progress you can pause the game. If you however pause the game at the moment you are being attacked the attack will continue to occur and your character will be killed while still in your pause screen.