Alpha Base (Halo Wars level)/Walkthrough
From Halopedia, the Halo wiki
Alpha Base is more like a "training level" to help you understand the basics of the game and introduce you to campaign mode. In this level, you are playing in the role of Sergeant John Forge and his Gauss Warthog, attempting to reassemble the Marines who were originally stationed at the run-down Alpha Base but were run off by Covenant forces.
First off, drive the Warthog to the arrow and keep driving in the direction of the arrow. Eventually, you will be lead to a few enemies, so press the X button and slowly take them down. After you kill them all, keep driving in the direction of the arrow. The blue arrow may sometimes lead you to a different place away from your objective, so change your direction once your sight is clear.
After you reach your first squad of Marines, they are pinned down next to three squads of Covenant forces. You can't control the Marines now, so drive Forge into a safe direction and kill the enemies one by one, with the support of your Marines. After they all die, the Marines will get in a Warthog, giving you two Warthogs to toy with. Be careful with the 'hogs, because they will have only 70% and health when you save the marines.
Keep driving in the direction of the blue arrow. Every now and then, there will be a few bumps in the road (pairs of Covenant squads); kill them by making Forge attack one squad and the Marines to attack the second. After a few Covenant encounters, you should reach another squad of pinned-down Marines. Remember to kill everyone of the Covenant if you want to kill a hundred grunts and get a reward on points.
There are much more Covenant in front of you this time, so take the same strategy again by having Forge and the Marines attack different squads. Once you defeat them all, the additional Marines will become usable, but they will now act as infantry, and will not have a warthog. Use your healing power to replenish your army's health, or they may die on the assault on Alpha Base.
Go into the same process, follow the blue arrow to the base, killing a few Covenant on the way. If you want the skull, kill every Covenant you meet.
After a while, you will reach a Covenant Anti-Vehicle Barrier with a few squads of Covenant guarding it. Your Warthogs will not be able to pass through it; you will have to order all of your foot soldiers to pass through it. Kill all of the Covenant first, and then attack the power core with your Marines' Y ability (Frag Grenades). Once the power core is broken, you can order your other units to pass through the barrier. Keep the Warthogs close to the barrier when your infantry is destroying the core.
Keep following the blue arrow. Your next stop is the base itself, so be prepared. Let your Warthogs pass through first, and use your Marines to kill the enemy infantry.
Once you enter the base's interior, you will find Wraiths in the middle of the base. Do not waste time attacking them, as a Shortsword bomber will kill them for you. Kill the squads of Covenant located at all four corners of the base. Send your squads one at a time and kill the enemies quickly with your Y ability. Once you kill all of the Covenant, your mission will end, with Alpha Base online and running once again.