Use the minigun on small zombies
The minigun can quickly take down lots of small zombies at a time. If the minigun's meter is nearing it's limit, take a break with it.
Throw grenades at big zombies while dealing with small zombies
The minigun makes you move very slowly. You can deal with this by throwing grenades at big zombies that get too close to you, which will stun them for a useful amount of time.
Wait until all or most small zombies are dead until you fight the big zombies
If you fight big zombies while there are many small zombies alive, the small zombies will have an easier chance at overwhelming you.
Use the rocket launcher on big zombies
The rocket launcher can dish out a ton of damage, but it takes a while to use. It is the perfect weapon to use on the big zombies still alive at the end.
Take advantage of barrels
Barrels can be used to take out a ton of zombies. Shoot a barrel when there are lots of zombies next to it. The small zombies will die and the big zombies will be stunned for a second.