In the starting area, you can find a case containing a Hologram. Equip it if you want then follow the path and go through the door. There are a lot of covenant forces waiting outside. Take out the forces you can see first before moving in and clearing the next section.
Continue forth until you catch up with some survivors. Follow the armed guards to the landing bay and take position. There's a turret there that you can use though you may want to let go of it and take cover in case your shields are depleted. Once you get the controls for the emergency barricade becomes available, clear the remaining enemies as fast as you can and activate it.
Once the shields are raised, continue to the marked door and clear the enemies there. Continue and regroup with the other soldiers and continue clearing the area. There's a plasma cannon you can use and detach so grab it and fight through the enemy resistance. Continue upstairs to find more enemies.
After taking them out, enter the next door and follow the corridor. You'll find a scientist along the way that will give you a Thruster Pack. This armor attachment enables you to burst to a short distance. You can equip this or just hang to your hologram. Head to the next room and deal with the two Covenant hunters. To get an achievement, you have to defeat these hunters using only the sticky detonator. Refer to the video below for more information.
Give them the stick
After killing the two hunters, open the door where the doctor is located. After the scene, head out to where the Composer is then make your way to activate the station defenses. Once you've entered a passage, wait for the soldier to open the path for you then continue forth. You'll come across a plasma cannon emplacement; take the enemies out and detach it. Use it to easily clear the next room.
Continue forth until you find another group of survivors. Join them and one of the scientists there will open a door with some Auto-sentry attachment inside. Grab one as it can really help you deal with enemies offensively. There's also a Sticky Detonator behind that will be very useful against clustered enemies.
Head to the next room and engage more enemies. The Auto-sentry will be quite useful here. Just be careful of the elite covenant that has the plasma blade as it can instantly kill you. Kill the next few waves of enemies and the next door will open. Head to the defense grid controls and take out the two elites there. Take note that one of them may be in stealth so deploy your auto-sentry to be safe. Once done, place Cortana in the panel to activate the station's defense grid.
Before heading to rendezvous with Dr. Tillson, check the small armory in the middle of the room to resupply then follow the path. You'll eventually reach a heavily guarded hallway before the gate where you must go through so deploy your Auto-sentry and lob some grenades in the middle to take them out easily.
Continue outside then ride the Mantis. Fend off the next few waves of covenant forces until your next objective is marked. Head to the elevator platform next. Disembark from the Mantis and activate the elevator from the panel inside the room. After seeing the composer gets taken, open the elevator door for a scene and to complete the mission.
Defending the composer