This one might be tricky. The ending fight is split into 6 phases:
1st phase: 4 of the turrets will shoot rockets and groups of enemies come out of both doors.
2nd phase: 2 of the turrets will shoot rockets and groups of enemies come out of both doors.
3rd phase: A zombie ball will appear out of the left door.
4th phase: 2 Franken appear
5th phase: 1 Franken and a group of zombies will appear.
6th phase: The boss will appear.
Note that if you die, you will need to start over and do everything again.
To beat the boss, you need to hit him with your ranged weapon first and then hit him with your melee attack from either one of the sides.
That sounds easy first but you'll see that it will be fairly hard since a massive amount of zombies will be spawned by the boss and worst of all: If you don't watch out, he will knock you down and instantly drive you to death without you being able to to anything.
Try to stick with the powerups.. one of the best ones is the zombies disguise - everybody will ignore you - including the boss, which is fine since you will have plenty of time to land an attack.
Good luck!