From Halopedia, the Halo wiki
The third level of Halo Wars takes place in the interior of the structure, where Sergeant Forge and Anders are ambushed by a platoon of Covenant. You are in control of two Grizzly Tanks, in which you have to fight your way towards Forge and Anders.
You begin on the outside of the Relic, where two Pelican Dropships place you in position to fight. Begin shooting the outer guards with your X button, and saving your Y powers toward larger enemies, such as Hunters and Sniper Towers. The Y ability of the Grizzlies is a powerful barrage of large missiles, which are very effective on pretty much every enemy coming your way. The Grizzlies can take a lot of damage as well. The level is mainly about using your Grizzlies together to kill all of the Covenant and work your way toward Forge and Anders.
Keep killing Covenant with your Grizzlies, using them together to kill the enemy effectively. The same strategy works in every level. When you have multiple units, make one attack one enemy, and the other attack another.
Simply keep killing any Covenant guards that dare to step foot in your path. Once you get to Forge and Anders, you will receive many more units, including a few marine squads and both Forge and Anders. Although you have so many squads, your Grizzlies continue to be the most effective.
After the Covenant destroy one of the light bridges in an to attempt to block you in, Anders mentions that she can hack into another one and reactivate it. Bring all of your units towards the system, and make Anders hack it.
It will take one minute for her to finish it, so be prepared for a large amount of Covenant, and keep in mind that the Grunts have large explosives. Make all of you marines surround Anders, and make your Grizzly tanks guard her as well. Kill all of the Covenant that try to stop her until she finishes the job.
Once the bridge is online again, you will be bombarded by even more Covenant. You get to have some fun with them, and if you make one of your units turn the bridge off with some of the enemies on it, to will kill all of them on the bridge and unlock the Covenant "Hot Drop" achievement.
Keep killing the guards and advancing toward the LZ. Follow the blue arrow and destroy all enemies with your Grizzlies. Once you reach the exit, various Hunters will attack you, so be prepared to use your Grizzlies Y ability to bombard them.
Once you reach the exit, make Forge and Anders go to the flare and be extracted back to the Spirit of Fire.