Functional Loadout
You have complete control over what Weapons you take into the Crucible so regardless of what mode you are playing it is important that you have a functional loadout. This would ideally mean taking a weapon that can deal heavy damage up-close and a long range weapon such as a rifle for dealing with targets at a distance.
Stick Together
Don't be a lone wolf, always try to travel with at least one teammate as doing so will make it difficult for the enemy to outgun you. Remember that it is almost impossible to win a 2-v-1 combat encounter if you are on your own and you do not have a Super or power weapon available.
Reviving Teammates
Avoid starting the actions to revive a teammate or interact with an item while enemies are nearby because if you take damage while doing so it will cause the action-meter to reset.
Have a Strategy
A good strategy is to focus on holding points, prefably the two points that are easiest to defend, with the shortest, safest route between them.
Be Patient
Recklessly charging into battle will get you killed in no time, you will have more success if you are patient and wait until you know that you will have the advantage when you rush a target.
Long-Range Support
Make sure when you play Countdown mode that someone on the team can provide long-range support with a rifle.