The only reason I don’t find it to be an exploit is the enemies still can, and will, damage you. Much like with Sledge and the doors. Other than that, if you thought your vehicle was fragile when fighting him in Playthrough 1, then just wait for Playthrough 2 where your vehicle is now wet tissue paper on top of just being tissue paper. You can expect to finish that fight on foot. Of course, it is very possible to finish on foot (Rocket Launchers are the easiest suggestion, while the Cobra and Volcano are pretty reliable for one-shot kills).
With the mission to kill 8 vehicles just park at the top of the hill where the Revolver pieces are collected. If you look in the direction where there are two garages for Bandit Runners (otherwise known as next to a small bandit camp with 2 red chests and a fuel tank for another mission) you can snipe all enemy runners from there, if you have a high elemental multiplier Sniper Rifle it is a one-shot kill in a lot of cases.
Let’s assume you have bad gear. If you hit an enemy runner, then it will come towards you, but they will never drive up that hill. You can simply man your vehicle’s turret, lead your shots, and very few of their attacks (machine gun or rocket) will actually hit you. You should never, ever die doing it this way. Once 8 have been destroyed no additional enemy runners will spawn. There may still be some stragglers running around, but you can always use that same hill to kill them by leading them there if need be.
Once that mission is over, and before you kill Mad Mel, no more runners will spawn in the area.
That just leaves the very few you deal with prior to that mission. When you first drive into the area there are 2. One of them usually parks further away, the first one. The second one comes more up the road toward you. It is possible to simply lock on to the second one and destroy him in a few rocket blasts before he does serious damage. The one further away usually will not get on top of you until the first one is dead. This might actually be the most difficult vehicle-to-vehicle combat in the area.
The next vehicle of interest is after you hit the 2nd electrical switch for the Fast Travel. Always have your car parked very close to the rock you jump on to reach so you can quickly jump in. The enemy will frequently get distracted by the Scythid, making this an easy fight.
After you activate the Fast Travel two more Bandit Runners spawn. Rarely, they will get stuck around the garage they spawn from. More commonly they will drive circles around a very large rock where the 2nd electrical switch is. My advice is when one of them circles around the backside of the rock you quickly destroy it, which is easy because for some reason this one usually just rams into the rock when you shoot it the first time. Instead of attacking you the second one circling the rock just parks itself on the other side. Once you destroy the first one, simply start nailing the parked Bandit Runner on the other side.
After that one is destroyed there are no other Bandit Runners to address until you fight Mad Mel.