This mission begins at the bar where you first met Mr. Cheng and his translator. It seems that they have gotten someone else to do business with, which has ticked Trevor a lot. Start heading over to the O'Neil Brothers' farm and when you get there, climb the hill to the vantage point.
Once the short scene plays out, start by moving up to a spot where you can get some cover. When you do, start opening fire on the enemies outside of the house. If they start closing in on your position, move to another spot and always remain in cover when you're not shooting. Don't forget that you also have your special ability, which helps in mitigating damage.
Make your way to the house and then go inside. There will be more enemies here, so be smart and use cover to help eliminate them all. You want to get to the basement, where the meth lab is located, so once you find the door, head downstairs. There will only be one enemy down here, who begs for his life, but shoot him.
Once the whole house is clear, grab the jerry can in the basement and start pouring gasoline. There should be a trail highlighted on your map, which indicates the general path needed for the gasoline. When you get outside, shoot the gas to ignite it, which will make the house go boom! Get on the vehicle and just drive away from the farm until the mission ends.
