What’s the optimal racetrack to use for the Rocket Manski achievement

Only melee weapons. So, any usage of any rifle or pistol (especially when you get incapacitated), including the grenade launcher will deny you of the achievement, even if you don't do any damage.

Chainsaw is a melee weapon, too, by the way.

Throwing molotov, pipe, bile jar or shoving zombies around (even killing them with a gun only by shoving) will have no negative effect. Also interesting is that usage of machine guns and stationary turrets won't cancel the achievement either, since they don't count as guns, but part of the environment.

HOWEVER, if you fire one bullet, you have to restart the campaign ALL over. Dying and repeating the map will not reset this (have actually tried it).

The key is that speed of zombie spawning is somewhat controlled by how fast you kill them, so try to make it as slow as possible. Potato mines are great early on. Snow peas are also good at doing slow damage. And of course you can just throw up walls for them to slowly eat through. Buy yourself enough time and you should have no problem getting enough sunflowers going.