Hands down, this has got to be the easiest way to do this.
Here is my version of the above solution.
First forget Arena B ( why make it hard on your self)
Second: you need molotovs and meat bait.
Join the arena by yourself. if you do, no one will be able to join and you will be alone. Pick the easy Arena (A). As soon as the first wave starts, go the right and you will see 2 barrels. walk towards the farthest barrel so the zombies will spawn further down the path.
Simply wait for them to get close enough, throw a meat bait at the barrel. This will keep them where you want them and then throw the molotov. You should net at least 2 or 3 kills. Then exit the arena to reset barrels and do it again
If you time it right after you throw the first molotov and turn around there will be a zombie coming the other way and will pass right by the first barrel.You could technically throw another meat bait at that one and molotov or just try to throw only the molotov netting you another kill. Point being, there are probably hundreds of variations to this and probably some ideal places to do this in the campaign. Arena's are easiest because you have the same setup every time.