Flying is definitely the stat you should pour your points into since it takes over 16 hours to max it out just flying regularly (not even joking 1% increase per 10 minutes flying). The rest there are a few good ways to level them up that are made easier if you have a macroable keyboard.
Strength: Can’t be macroed, but as has been mentioned some missions won’t result in you getting wanted stars. Load one of these, go to the pier or the cement path by the beach, and just punch everyone you see. I usually use Flood in the LS River as the mission simply because it’s easy to clear out all the enemies from the bridge just south of the mission marker. You get on average 1% per 20 punches, so it’s going to take a while.
Stamina/Stealth: Start up an invite only session, go to the golf course, stand in a wide open area, hold down A and W on the keyboard then hit Shift (to run) or CTRL (to sneak). As long as you hold down A and W you will run/sneak in a circle. This is one of the macroable stats, and it shouldn’t be too hard to figure out which keys to set to hold down for the macro.
Flying: Can be macroed and left overnight, get in a helicopter, hold W until you are at the height ceiling, then hold down A and tap 8 on the numberpad every half a second or so. As I said it can be macroed using Razer Synapse or a similar piece of software and left to run overnight.
Driving: Hit jumps, as many as you can as often as you can. If you have a friend who can get a cargobob have them pick up the car you’re in and drop you from max height. If you land on your wheels and don’t blow up you’ll get 1% for every second you were in the air. You can also just drive regularly and eventually you’ll max this one out.
Shooting: Get a friend, go to Ammunation, do the minigun target grid challenge. Be sure to set the number of challenges to 5 for this one. If you are sure to alternate wins so it takes all 3 rounds per challenge and you go all 5 challenges, this should get you at least 1 full bar per challenge run.
Lung Capacity: Swim underwater. That’s it. Just go out to the beach and do it. It takes forever to finish, but a trip around the whole island underwater should get you there, or at least very close.