Frozenghost001's solution pretty much covers the locations, rewards and tips for this achievement, but if you are having trouble with this by yourself like I was (I think I was stuck at level 4 or 5 of the activity in the Apartments District), doing this activity in CO-OP makes it exponentially easier.
So why should you find a co-op partner to do this with you?
1-Works towards your
2- Get this - There will be a second player in the activity chucking an infinite supply of molotov cocktails at peds and cars giving you the same time bonuses as the driver hitting people/blowing up cars! I can't stress this enough! You have someone with you chucking fire bombs at people and cars while you're driving giving you gigantic time bonuses and a huuuugeee room for error. I honestly think that if you have played through this game once and are familiar with the Trailblazing activity, you CANNOT fail with a co-op partner unless you do it intentionally or glitch into a wall or something. That's how easy co-op makes this activity and it basically removes most of the individual skill aspect of this achievement, and only makes it kinda time consuming (maybe 25 min total).
Me and my co-op partner managed to tackle all 12 levels of this activity in one shot, 0 failures.