APB Reloaded

Okay so a few ways go about getting this achievement, the top two being having someone help you or just random luck.

You acquire grenades by following the in-game tutorial.

If you have someone help you that can either be with a friend or you can use the in-game chat to try and communicate to random players. Text chat can be done by pressing start and finding the chat option on the left side.

Now straight from the get go you can't just fight anyone on a server. You need to be in the same mission. Multiple people can habitat the same server but be doing different missions. Now normally you can search for a random mission by pressing up on the d-pad but this would guarantee that you and your boosting partner on the opposite faction will get put into the same mission even if you search at the same time because missions will start for other people with less than the maximum amount or people will drop out and that means either one of you could be put into a already in-progress mission while the other player gets put into a different mission.
Luckily Enforcers (these are the cop players) can force a mission on Criminals by "Witnessing" them doing a crime. The Criminal need to steal a car and while they're doing that the Enforcer needs to aim their gun and when their reticle turns orange press down on the d-pad and this will start a Bounty mission. Ignore the objective.
Instead have whoever is going for the achievement be outside of the car while the other person throws their grenades with . Grenades are very weak, so you may want to shoot the car a little bit till the hood of the vehicle is visibly dent, it's also a good idea if the person that is going to get blown up is in a normal car as opposed to something like the armored van or a cement mixing truck.
Once the first person is dead the other person can jump in a vehicle for the other person to blow up if they need the achievement too. Then either player can go finish up the mission.
Make sure not to be in a group with other players unless they're boosting with you as they may get in the way of things.

Now if you're not going to have another person intentionally sit there while you throw grenades, things become much harder. While this is the most obtained achievement out of them all it's really not that easy. It's more random than anything. Like I said above grenades are super weak and you can only carry two of them. You can resupply yourself with more grenades if you run to a Joker Ammo machine but that may not always be possible so you can also use the Field Supplier with the left d-pad which is a character mod that you acquire from doing the tutorial. The Field Supplier goes on a cooldown aftter use so keep that in mind.
Here's everything else you have to deal with:
-Grenades have long timers (you can speed them up by cooking them from holding the before releasing the button and throwing it)
-Grenades bounce around
-Grenades have a small blast radius
-Cars can take a single grenade hit
-Cars can drive away before you can get a grenade off or the person can exit the vehicle
-Team mates can blow up the vehicle or the enemy driver can damage their vehicle by driving into things
Basically ideal scenarios will be if a person messes up driving and boxes them self in a tight area or they get boxed in my another driver a less likely but still possible event is that a passenger is sitting out of a window with their gun drawn. I've been killed the first two ways and ended up getting lucky and getting my achievement from the third. When someone is sitting in the passenger windows they have to do a fairly long animation of getting back into the car before they can get out. So do your best to distract them without dying, put some damage on their car, cook your first grenade and then for added measure quickly throw out your second. Worse case scenario you die but one or both grenades will finish them.

If you go for this via legitimate means randomly you should gain some progress towards this achievement too:

I think this might be currently glitched but in a good way but I'll go into more details about that in that achievements own guide.

Now if all else fails and you absolutely can not get a kill while someone is in a car with a grenade you can always play the game and level up your pledges with the different NPC factions and then upon reaching rank 85 you buy a grenade launcher. This will take a considerable amount of time but it should work better than normal grenades.