The Guide to Fighting with a plan
This entire guide will eventually detail on the command and mainly the boots on the ground level of fighting against xenomorphs. Both on the offensive and defensive, this guide will show you how to effectively fight on the marine team, the mentality you need as one, squad composition as well as several generalised and basic loadouts for each squad marine type.
Remember, this is a GUIDE and not concrete rules you need to follow. Improvise, adapt, overcome.
The Basics
Communication is key
The number one rule in team work is communication. Talking with your fellow marines is vital and more importantly, relaying information to the CIC. Command's reach of information is through the Overwatch consoles and the tactical maps. This allows them to view the battlefield through a marine's helmet if they are even wearing one. Though, highly recommended you do wear a helmet. Some marines choose fashion over function. To each their own. Information is very important. I can't stress this enough knowing that I've seen marines lose rounds when no one told command, their squad or anyone that could help. That something was happening and they needed to act. Relaying your positions is very easy. Examine the Squad/Team Leader Pointer to see your current location.
You need to relay battlefield information to CIC in order for them to make an informed decision.
Are the xeno's charging through T-Comms on Solaris Ridge's LZ 1? Tell command, they'll get on the announcement system to drag the marines into repelling the incursion.
Is there a tunnel dug right outside the LZ where no one but you saw it? Tell a combat technician to grab some C4 to blow it up.
Did you see a lurker stalking the path to the frontline? Tell your squad that the road to the front is dangerous and buddy up.
Staying on radio silence is a terrible idea. Command and more importantly, the marines around you, can't make informed snap decisions that will help your team as a whole.
Offensive Doctrine
You shall have no fear, you shall never fight alone and you shall fight with righteous fury.
Defensive Doctrine
Rally to the flag, stand your ground, for they shall not pass.