sometimes in Death Valley when you try to glitch in last round you failed or the zombie killed you after the 4 bosses came . It`s not the end you have another chance , just follow this tips :
1: stronger 2 players try to go the mid as fast as they can and 2 stay up. let`s say the 2 will go down (A) and 2 stay up (B).
2: B will press "x9" to grab the attention of the bosses, and keep pressing it .
3: A will try to take 1 boss with them in mid and kill him. B must keep to press "x9" to grab the attention of another bosses .
4: when A finish 1 boss then try to take 1 ealse untill last boss and all go to mid and fuck him .
I hope I helped you and sry for bad english .