This can be done solo, with AI survivors.
1.) Play Survival mode, load up "Dark Carnival: Stadium Gate".
2.) From the spawn, pick up and carry one Molotov over to the scaffolding in front of the gates. Swap out for the pipe bomb. Return to the spawn and pick up your second Molotov. This enables you to have both Molotov's in reach while on the scaffolding.
3.) Start the round and return to the top of the scaffolding. Watch for the red-hued clowns both towards the right of the gate above the Porto-johns as well as back towards the spawn.
4.) Wait for zombies to occupy your AI allies, until a larger horde successfully drops into the area. Let 'er rip. With two shots, you have a pretty good chance at nailing a clown, unlocking your achievement.