|After finishing The Big Score, switch to Franklin and go to his house in Vinewood Hills. While there, someone will come to the door and you'll see that it's Devin. You will be presented with a choice between three different outcomes; a choice of A, B, or C. To start this mission, choose the C option.|
After the conversation with Lester is over, head to his house to start the plan to save both Michael and Trevor. Your first task is to go pick up Lamar from his house, then you will need to head to the foundry. Once you get there, head inside and meet up with the others.
Get into your respective positions and when ready, open fire on the enemies below you to start this fight. When you see mention of Merryweather on the upper catwalk, switch to Michael and use the sniper rifle to take them out. Switch to Trevor after that and kill the Merryweather guys that come in near him.
Switch back to Franklin and keep killing enemies as they come in, until Lamar chimes in and says he needs some help. There are two exits you can take to get outside, but it's easier to make your way to where Trevor is and use the exit in the office there. Continue killing enemies outside with Franklin until you have to switch back to Michael.
Once in control, you need to head towards Trevor's position, killing anyone in the way. He will be on the ground near the ladder, so get to him and you will be able to switch if you want. There will be a few more waves of enemies inside before you will be ordered to go outside, so take the same exit you used as Franklin.
When you get outside, take care of the enemies in your immediate area, then head on over to the other section to find more enemies. Eventually, a helicopter will show up, so if you're using Michael, use his ability to pick off the pilot or blow it up with a RPG. With everyone dead, regroup by the main doors and this section of the mission will be over.
It's time to tie up some loose ends, with three targets right off the bat. Franklin will go after Cheng, while Trevor goes after Steve Haines and Michael after Stretch. You can switch between all three characters and take them out in any order.
Harold "Stretch" Joseph
Michael is the one going after Stretch, so follow the GPS route to his location. It might be a little difficult to locate him, so something like an explosive would work well to kill him and the rest of the Ballas in the area. He is the bald one, so kill him any way you want and escape from the Ballas (or kill them all).
Steve Haines will be at the docks, in one of the cars of the ferris wheel. When you approach, you'll see a short cutscene. A sniper rifle is ideal for killing him, as it will give you enough time after the fact to aid in your escape from the cops. You only really get one shot, so make it clean and don't get too close. A two-star wanted level is your reward for killing Haines, so lose it and go after the next person.
Wei and Tao Cheng
This one can be a little tricky, as when you approach, they will get in their vehicles and take off. Luckily, Franklin can slow down time while on a vehicle, so equip some Sticky Bombs (if you have them) and use the slow down to toss one on each vehicle. If you don't have any, then use Franklin's ability and shoot the middle one at least until it explodes. Clean up the rest of the Triads and it'll be time to move on.
No matter the order you went in above, Trevor will be the one going after Devin. When you arrive at his house, you see a warning that there are Merryweather guards around his compound. You can either sneak around killing the guards, or just go in with guns blazing.
It's probably easier to just go in without sneaking around, so go around and pick off all of the enemies. When they're all dead, go to where Devin is located and after a short scene, it'll be a long ride to the rendezvous point. Once you get there, there will be another scene and then all you have to do is hold forward on the left analog stick to push the car over the cliff.
Enjoy the ending, for you have finished the main story of Grand Theft Auto V!