You just have to keep drive-bying them. It took me like 20 tries!Sometime you're just gonna get lucky and have a few extra nines scattered about. Also, try to shoot up the vans that are after you because it'll slow you down usually and because you are shooting up the vans, it attracts the hoods and you can nail some more nines. there's no real trick or loophole to this mission, you just have to be efficient and lucky