Consider this line intro.
1. Use assault rifles
They do heavy damage, and are good for short-mid range combats.
2. Don't try to find enemies, let them come to you
Remember, your goal is to last longer in the game, not to kill many enemies.
3. Use adrenaline shots once you are in top 20
It increases your hp to 150 than normal 100.
4. Always Go for headshots
Use scopes to make it bit easy.
5. Use silencers
...so that enemy can't spot you. Especially useful with snipers