IMPORTANT: You can only do the firefight achievements on the DEAFULT SETTINGS (standard starting resources ect).
Difficulty can be any.
Build Marines, Snipers and Turrets for the first few waves. If you're playing as the Banished build Unstable Barrels, they are great at taking out large numbers of enemies.
Then try and capture and hold the mini bases. Hold them with Turrets, Walls & Siege Turrets.
Now after about wave 10-15 Artillery, (such as Siege Turrets, Kodiaks and BlisterBacks) are extremely useful as they can deal large damage to clusters of enemies. And will avoid direct engagements with the enemy's ground forces.
Your army should be made up of Wolverines and Kodiaks or if you’re playing as the Banished Reavers and BlisterBacks.
After around wave 35-40 the enemy units will become way to strong so to earn more points (if you still need them) you will have to funnel the enemies.
Jack Rabbit with 'Support drone' upgrade can provide a great line of sight for Artillery.
Use Spartans to hijack the powerful Enemy Vehicles (Grizzlies, Vultures and BlisterBacks).
Direct Air has a 1 in 3 chance of Coming on the 4th Wave of a UNSC or Banished Round, prepare for this.
Try to keep your population level high by taking and holding the small bases.
Siege turrets are extremely useful as like Artillery, they can damage large clusters of enemies.
If you are facing infantry waves put 3 Snipers in a Garrison Barricade.
Nightingales & Shroud's can Cloak the Terminus with their abilities.
Chase the Bonus cores to a point, don’t waste units trying to get them, but don’t just ignore them. They will provide Supply, Power, Health Packs and Veterancy. Veterancy is particularly important as you get a 3 Veterancy for just a simple pick up.
Keep an eye on your ally's Supply and Power, send them resources if their low & if you can spare it.