Easy and quick way to get this is to setup an online cruise with a friend and propose a Splitbase Capture the Flag match on Century City Garage.
The flag always spawns in the centre of the car park on one of the levels, you need to be at the same level or above the flag. Driving through the smoke will pick it up even if it's on a lower level. The delivery points are very close and you can jump off the car park at any level through gaps in the walls.
There are two ways to play. Either take it in turns to deliver the flags and you will collect 9 and 10 each in about 8mins. Or one collects 10 flags (takes about 6mins) whilst the other drives around the nearby freeway collecting mileage towards one of the other achievements (5k, wheelie, two wheel).
Repeat the match and delivering 100 flags won't take too long.