Why not place that using linked Campaign Maps and put in lore for the game, Put some spicy extra challenges for the bonus
Every step on the story clear gives YOU unique prizes for clearing the step
Complete the round with 50% hp or above
Kill <NaN> of zombies
Be Alive for 5Minutes or more
Dont die to a zombie projectle
Complete the round with a specific setup( any combo possible)
Completing those it will be rewarded for you AND IF HAVE, for the party.
The rewards can be some cash, xp , items (themed when have) or any other.
This should be good for who likes the challenges, and play for the risk-return.