After getting to the "TT" icon, there will be a short cutscene. You will then be given 5 and a half minutes to burn all the weed fields of The Truth using the flamethrower given to you by him. If ever timer runs out, Truth will get arrested, and a failure will result. So, to make it quick, simply hold the fire button while you release fire. The Truth will work with you, but after about 1 minute, 10 seconds will give up. You'll be required to finish it while he waits for you in his Mothership. After the fields are destroyed, approach the Truth to get a rocket launcher (which he was going to turn into a lamp) to destroy the helicopter.
Use your newly acquired rocket launcher to bring down the chopper into flames before the time runs out. Ensure that it doesn't hit the Mothership or you will fail. In any case, when the chopper is nothing but old rubble, jump into the Mothership and drive over to the garage you won from a race with Catalina and Claude. You will no longer have the timer to worry about, nor will anything chase you. Just take it nice and slow to the red marker and it's finished.