The first part of this mission has you flying a fighter plane across the Didact's ship's hull. You'll be doing some serious maneuvering to stay alive here, which means you likely won't be looking at this walkthrough while you're doing this, so what I say here won't be of much help!
Along the way, there will be turrets that shoot at you that are good to destroy when you can. There are also these balls of energy (or whatever) that you have to shoot and destroy at some points to proceed.
Still alive after all that insanity? Good! In this large area at the end of the path, you'll have to destroy all of the guns so Infinity can punch a hole for you to get inside the ship. Nothing too tough here, just either avoid or destroy the turrets and you should be fine.
You'll end up inside the ship with the bomb. Head forward through the doors. Be prepared for loads upon loads of Prometheans to be thrown at you this mission almost nonstop. Hey, at least there's no Covenant this time, right?
Watchers in the next room. These ones can spawn defense turrets, so take them out quickly. Head through the door on the left afterward.
As you advance through the next few rooms, you'll be attacked by tons of Watchers and Knights and some Crawlers. Get used to the Crawlers, they're going to be your worst nightmare this mission.
When Cortana tells you to head through the door on the left, go through it and into the beam that will send you upwards.
At the top, insert Cortana into the console, then run through the portal she opens up.
Crawlers will attack you again, make your way through the next portal Cortana opens for you.
Fight your way through the swarms of Knights and Watchers as you make your way to the next portal higher up.
In the next room, you'll have a minute to yourself to breathe and relax before the final plunge into insanity for the rest of the mission. You even have plenty of weapons to choose from in here! When you're ready, head through the portal.
Next room: Knights, Crawlers, Watchers, the usual. Fight through them to the next portal.
This room is where things really start to get real. Head forward and take out the Prometheans here.
You'll now have to defend this area while Cortana finds the Composer. Get ready to be assaulted by more Crawlers than you can shake a stick at.
It's good to take this lightrifle ammo, as it can usually one-shot Crawlers. Fight for your life. Eventually Cortana will take control of the defense turrets to give you a hand, which will help a little.
Eventually Cortana will have you remove her from the system. When you do this, head across the bridge to the marked door while doing your best to push through the massive amounts of Crawlers coming at you. Get through the last portal!
After going through the portal, head into the beam to be teleported up higher to the Composer.
Head forward, launch forward with the grav lift, and head to the console in front of you and insert Cortana into it.
Head to the right, take the grav lift to the structure ahead of you, and start fighting through the hordes of Prometheans as you head upward towards the objective marker.
At the top, press the switch to activate a light bridge across and continue on.
Prometheans will continue to assault you like crazy. Push your way through to the console up top, insert Cortana into it and let her do her thing. Afterward, jump down and take the next grav lift marked on your HUD.
Again, Prometheans will assault you almost nonstop, but as long as you're patient and don't waste your ammo, you'll be fine. Head up the structure to the next marked grav lift and take it to the next structure.
Again, at the top of the next structure, push the switch to activate another light bridge to continue on.
Once you reach this dead end, you'll have to take down all the Knights here. Once you do that, the console will raise up and you can finally insert Cortana into the console and move on.
Jump down and head into yet another grav lift to be sent to yet another structure. More Knights will attack, but at this point you should be able to handle anything as long as you're okay on ammo.
Once the Knights are down, head into the beam and book it across the light bridge to take on the Didact.
At this point, all you have to do is follow the on-screen button prompts. It may take a couple tries, but it's not bad when you know what's coming.
And that's it, you beat Halo 4! Congratulations! Watch the ending cinematics and breathe, it's finally over....for now. Until Halo 5. Thanks for reading, hope this helped you a little!