Don't start the mission until you fill up Michael's special meter. Just take his car out for a spin because Michael's special meeter fills up when you maintain high speeds in a car. Once you have a full meter, start the mission.
I don't like to advance through the cutscenes so I hate jobs that have a Mission Time criteria. You can't meet Mission Time unless you skip all of the cutscenes.
The hardest part about Predator are the headshots because they are hard to get without Michael's special and waiting for the shot eats up Mission Time, but you can split them up between Michael in Franklin.
The headshot on the guy in the water will be pure luck. Even with Michael's special on, the dude in the river will still swim pretty fast. Best advice I can give you is to turn on the special, lead your shot, and hope the target swims into the bullet.
Second headshot is easy, but it takes the O'Neil brother a long time to clear the tree line so that you can get a good shot. Be mindful of the mission time and try to get the first two headshots as quickly as possible.
Third headshot is easy. Switch to Franklin and have him chase down the last guy. Once you find where the last O'Neil brother is located, arm one of your better pistols and shoot him in the back as he runs away. Keep your gun trained on him and line up your headshot as he gets up off of the ground.
If luck is on your side, you will have only fired a total of five bullets.
TBH, this is one of those missions where it is better to be lucky than it is to be skilled. You may have to replay it a couple of times later on to meet the Gold Medal criteria.
Fly back to the airfield as quickly as you can and remember to advance through the last cutscene.