Please note this guide is for Standard Marines, as other roles should be focused mainly on their job.
Roundstart
- Acquire the M240 flamethrower with three spare tanks from your SL.
Put the three spare tanks in your ruck.
Get a M4A3 sidearm and belt as a backup weapon.
Get the essential fire extinguisher to put out the entire squad of baldies you will light on fire. Click it to turn off the safety.
Fill the remaining room with first aid supplies from a NanoMed.
Wait in the RO line and get a magnetic harness for your M240
Your primary role is area denial, not getting kills (except for that really dumb Young Runner that pounced into your fire). You are a force multiplier, you make it easier for everyone else to kill the Xenos.
When pushing utilize the M240 to cut off Xeno flanks (such as holes in walls, open rooms, etc) so that they are concentrated or forced to flee, or to halt or slow a xeno counter-assault.
When retreating use the M240 to create a wall of fire to cover the limping Marines with a pistol firing wildly.
Remember that Ravagers and Queens are immune to fire; don't even bother, use your pistol while running away to safety. You are a high value target to the Xenos, so always have a buddy near you to save you.
UPDATE: With the addition of a Pyro Specialists and the subsequent nerfs to the M240 your role has shifted slightly, the Pyro now performs area denial more effectively than you will. Try to keep your distance from the Pyro to make sure every Xeno gets some.