This is the last story mission of the game, so make sure you are prepared before accepting to do it. Also, if you want to get the "best ending," you have to finish all of your Homie missions before attempting the mission, as you won’t be able to replay this mission.
Grand Finale, Part 1
As the last story mission of the game, you are warned about being ready for it, so before accepting the mission, make sure you have done anything necessary for your survival. This is especially important, since you can’t replay the mission, so do every single Homie mission (including the Loyalty ones) for the Saints & Sensibility trophy/achievement .
The mission itself is divided into three main parts, where you’ll be able to bring along a team of two others. The first team you pick will join you in the first part, then you’ll get to choose a second team for the second section and the team you didn’t pick will join you on the last segment. The teams consist of Pierce and Shaundi, Matt and Asha, and Ben King and Johnny Gat.
When you begin the mission, you’ll be sitting on the back of a flatbed, with the newly constructed key. Your job is to protect it from the swarm of Zin that will be chasing after you, and you’ll be armed with the alien rocket launcher, which is more than capable enough to destroy the vehicles, tanks and ships that will be coming to destroy the key.
There will be one point during the flatbed part where you will stop and have to take out a roadblock, then later on, Zinyak will make your truck disappear. That means the rest of your trek will be on foot, with your Telekinesis power being used to move the key. Pick the key up and sprint right towards the tower, where you’ll have to wait until CID hacks the door open.
Focus on the portals and Marauders first, as they tend to be the most troublesome, then concentrate on the rest of the enemies. Also, be on the lookout for the power suppression grenades, as you will likely need to use your powers with all of the enemies moving about. Once CID is done hacking, you can go inside of the tower.
The inside of the tower is big and it is about to get crazy. Follow and keep an eye on CID as he escorts the key to its destination, until you’re told to stomp the breakers. There’s three of these and all you have to do is get to the base, using your Stomp power. It is at this point that pretty much all hell breaks loose, as there will be A LOT of Zin attacking the key.
It’s important to keep an eye on the key’s health, as well as your own health. The key will stop once again when it hits a barrier, which is your cue to disable two generators. Once that is finished, the key will move once more until CID points out about some relays. This is where the battle can get very hectic, as now Wardens join the fray. Ignore them and destroy the relays so CID can bring the key to its destination.
Grand Finale, Part 2
You’ll be back on the ship and will have to choose the second team to join you. After doing that, head back into the simulation and go to the marked platform, where you’ll need to kill ten Zin. With that done, Kinzie will tell you to defend the blue portal in the middle of the platform for a total of one minute before you can move on with the mission.
Once it’s done, one of the two people you brought with you will stay behind to protect the portal. Note that you will be fighting mostly Zin and ships during the protection part. Anyways, move on to the next platform, where you will need to do the same thing, except you have to protect the portal for 90 seconds this time. There will be one more platform and portal, with you having to protect it for two minutes.
Unfortunately, you won’t get to admire your new, and large, friend, as the simulation is crashing. You now have three minutes to get to the exit, which is more than enough time. However, when you get to the first gateway, it doesn’t work and you’ll have to go to the next one. This carries on like this until you get to the third gateway, where it looks like the game locks up. Select "Continue" on the original title screen for Saints Row to finish this portion of the mission.
Grand Finale, Part 3
With the assault on the simulation ending, it’s time to go after Zinyak in the real world, with the team you didn’t pick joining you in this endeavor. Your first goal is to find the armory and there’ll be a lot of Zin in your way, so keep fighting until you finally get to the door. The only problem is that the door is locked, but your one Homie decides to try and open it, so you’re going to have to protect that person.
There will enemies coming from all three lanes, including Murderbots and regular Zin. Eventually, the door will be opened and you can go inside, where you’ll find some power armor. After donning it, you find out that you have the same powers as you did in the simulation and decide to find your own way into the throne room to Zinyak.
Once you gain control again, you’ll be falling down a large shaft, with lasers appearing throughout. One touch of these lasers and you will be fried, so do your best to dodge them (usually an opening in each pattern). A gate will close and you’ll have to find some power conduits to force open the gate. Destroy them with weapons and the gate will reopen, allowing you to fall down and eventually reach (watch the debris) the throne room with Zinyak.
Final Boss: Zinyak
Rather than fight you mano e mano, Zinyak decides to pilot a giant robot instead. It’s best to jump to the middle platform as soon as the fight starts, as Zinyak can hit hard and fast with his blasts, and you can use the pieces of the platform that are sticking up as cover. While doing this, keep taking pot shots at Zinyak and when his health reaches a certain threshold, he will fly to the central platform and erect a shield.
In addition to the shield, Zinyak will also summon a bunch of CIDs that will move around him and the shield. He’ll then begin making the CIDs come after you and after a few seconds, they will explode. What you need to do is use Telekinesis to launch the CIDs back into Zinyak’s shield. The problem is that there will be a lot of CIDs and regular Zin will start spawning from portals, so things can be pretty hectic.
When you are finished with that, you’ll have to disable some generators to finally bring Zinyak’s shield down, but there will be a lot of enemies now, making it hard to disable them without getting interrupted. Luckily, after a short time, your Homies will come in and start fighting as well, giving you some time to finally bring down Zinyak’s shield.
Once the generators have been disabled, you’re free to resume your attack on Zinyak, who shouldn’t have that much health left. When all his health has been depleted, he will be ejected from the robot and there will be a few quick-time events left to finally end his reign. Watch the ending and rejoice in beating Saints Row IV!
After the credits and cutscenes have finished, you’ll be allowed to return to the simulation and finish up anything you haven’t already done, such as Activities, side missions, collectibles, or just general mayhem.