The Assault On Arcadia
Level 4 - arcadia city
one of the most difficult levels in the game. keep your starting units alive and clear out as many covenant units as you can from around the map and from the garrisions. collect supplies with forge. don’t bother protecting cargo 3 because it gets destroyed anyway; focus all of your attention on cargo 2 and make sure that nothing even scratches the paint.
once you are allowed to build a base buy it on the grassy area where you started the mission. make 4 supply pads and 2 diagonal turrets to help defend your base. upgrade the base and make 2 reactors and an air pad; upgrade your supply pads too. pump hornets untill you can’t make anymore then buy advanced reactor for tech 3; buy wingman then sell a supply pad for a field armoury. research reserves and reinforcements then sell the field armoury.now you can either make more hornets, make some wolverines to help spawn kill dropships, or make some marines to put in the garrisons. it really doesn’t matter becuase wingman hornets should handle everything anyway, this just gives you something to do while the timer goes down.
make sure to save the game every 2 or so minutes incase you do something wrong.
you must keep the cargo 2 transport at full health at all times or the mega turret will destroy it!
Level 5 - arcadia outskirts
another difficult level. as soon as the mission starts go left down a narrow ramp. there should be 2 crashed warthogs and a sniper tower; put Jerome or douglas in the sniper tower then push onwards. put an odst in the reactor then head for the crater. garrison the spartans and infantry so they take less damage while clearing the enemies out. once thats done send the tank to clear the path to the second bonus reactor and put another ODST in there. on your base you want to make a baracks third and pump marines into all the garrisons you can. upgrade your base and supply pads as you see fit, but make sure you always have 600 supplies saved up.
when the mega turret spawns Jerome/Douglas will start shooting it. MAC blast while its building and that should bring it down to half health; send the tank there to help finish it off and you might want to mac it again because it has ulrta-high health. while this is happening spam marines and flamethrowers to help you defend.
buy the second base and get vehicle depots up on it aswell as a field armoury. upgrade tanks to powerturret then build some to go with your marine/flamethrower army. now you can either push on the covenant base or wait for omega team so you can spartanise your tanks then push.
Level 6 - dome of light
not as difficult as the last 2 levels, but it can still be difficult to get through. when the game starts fight off the units near your base then put Jerome in the closest air tower and Douglas in the one furthest away. put Alice in the bonus reactor furthest from your base and put a marine in the one at the back of your base. make 4 turrets and make the back ones anti-air and the front ones anti-infantry. now sit in your base and upgrade your units. make an army of tanks wolverines and flamethrowers and fully upgrade them; it might be an idea to get field armoury upgrades too. don’t call in any rhinos untill all the units you need are built and upgraded fully.
position your tanks on the walkway inbetween positions 1 and 2 with the wolverines and flamethrowers behind. call in the first rhino and the wolverines should kill all/most of the dropships and any units that get through will be cleaned up by flamers and tanks. call in the second rhino and your tanks should wipe out the covenant mini-base before it builds. for position 3 pelican 1 or 2 tanks to clear it; move the rest of your tanks to where positions 4 and 5 will appear and then pelican the rhino to position 3. when positions 4 and 5 are highlighted your units should kill the locust at position 4 and destroy the covenant mini-base at positon 5. move the rhinos into position and you win.
Level 7 - scarab
Its a trap! but its alright you can get through this mission. there are two ways to do this level; you can go around trying to take down all the power nodes while simultaneously trying to defend your base from waves of locust and hunters while living in constant fear that the scarab could break through that flimsy wall of ruins any minute, or you could glitch it.
there is a sniper tower in the middle of the map near a power node. as soon as the mission starts send the tank to the left of your base; there should be some honour guards behind a ruin. kill them then move the 2 starting marines up behind the ruin. send the tank to the other side to kill the locust there and clear the jackals out of the sniper tower. upgrade your marines to new blood and place them in the tower just before it researches. if everything went according to plan the scarab should be stuck shooting at the tower and not the marines. now move your tank and the rhinos to the scarab and lock down and shoot at it. you might also want to make a wolverine and a cobra to protect from banshees and speed up the process; respectively.
while you were doing all that you should have also put up 4 anti-infantry turrets and upgraded your cobras and marines. make 12 marines and 4 cobras then if you have any spare population make another cobra to lockdown in front of the scarab. split your forces so 6 marines and 2 cobras are guarding the right side and the same number guarding the left side.
now sit there and wait for the scarab to die while periodically MAC blasting it to help speed things up. there is a scripted event where the scarab is forced to shoot the ruins protecting your base at around 7 minutes in, this signals the start of the covenant counter-attack.
sometimes the scarab will stop shooting at the tower and then drift towards your rhinos and cobras. if this happens use the tank to draw its fire back towards the sniper tower and it should continue shooting it.