Yeah i keep running out of time too, my record is 145. I literally waited till there was 3 cars next to eachother and shot, they blew up and nothing. I already have that post liberated in this guide so thats a no go for me and every post ive done trying for this has no cars with 4 ppl in them. Im at like 12/34 posts done now.
EDIT: Just got it randomly in that Trial. I dont even know how but YES