Q:

How do I defeat 75 zombies within 25 seconds? 

In the Wild West area, for the first star on day 6, you have to defeat 75 zombies within 25 seconds.   However, there aren't 75 zombies in the last 25 seconds. 
What should I do? 

A:

You don't need to do this in the last 25 seconds, just any 25 seconds. 
There are several plants that can do damage to multiple zombies when assisted by
plant food. 
The best time is during a wave, put some bonk choys on row 2 and 4 right in the middle of the pack and give them both plant food. 

A:

I just keep trying and try to kill faster on last 25 seconds.  I did it. 
So basically just before the final wave I spend plant food to bombard them with as much watermellon as possible.  Once the final wave is out, I bombard them again with watermellon food.  Looks like I reached 75. 
The voted up answer is wrong.  Yes, you don't have to kill it at the last 25 seconds.  However, there are even less zombies on other periods.