Since I've been waiting to raise my strength but having no one to do it with I finally just went and tried to do it on my own, started a Gerald mission so there wouldn't be any cops. Heard it could be done with a bus but I found a delivery/cargo truck jumped up on it and attack to see if I could do it and stay on, it worked and got my strength to 40 before being knocked off because he hit something.
After searching for way too long for another I ended up down at the docks in the area where you take cars for Simeon. There was a waste truck, one of the ones that look like they're hauling broken concrete stopped at the toll booth if you follow the freeway over to it, parked a car in front and climbed on, first he exited the truck and wanted to fight but I equipped a gun (and unequipped) and he ran back into the truck, but didn't drive off. I just stomped for a few minutes and quickly made it to 100. It probably would have taken around five minutes to do if I wasn't interrupted the first time.