This activity is best done once you've wiped out all the gangs, since it's gang vehicles and gunmen that will hassle you the most when you're trying to bring back the hos.
Play through the story and eliminate all the gangs; once you've done this you'll have a couple of sturdy heavy vehicles to choose from: an APC (aka The Bear) and a Humvee-like truck (aka The Bulldog) with roof-mounted miniguns. Upgrade either one of these at a garage. Neither of these trucks are really fast, but they have decent speed, mounted weapons, very good armor, and three passenger seats. I used The Bear, but it's really down to personal preference.
With minimal gang resistance, getting the hos back to the drop-off spot is much easier; you'll get chased by a gang vehicle here and there, but your vehicle's roof-mounted minigun will make short work of any opposition. Any homies you have in the area will also help out by shooting at pimps and enemy gang members.
In the later stages when you have to make multiple trips, try picking up three hos at a time before returning to the drop-off point. If your vehicle has heavy armor, you won't have much difficulty driving to multiple pick-up spots; remember, assuming you control the map, there will be very little opposition outside of the pimps.
With all the gangs wiped out and while driving The Bear, I was able to complete Level 6 of this activity with nearly three minutes to spare and without having to change vehicles once; when I reached the finish line, my APC was hardly touched.
Note: if you want the APC but don't want to play through most of the story to unlock it, raise your wanted rating to cause SWAT and the FBI to arrive. Then merely steal an APC and drive it back to one of your hideouts.
The Bulldog can be acquired by completing the Sons of Samedi storyline.