run under his torso with a shot gun and fire away at the yellow sacks in him, then eventually you will see a cut scene where he picks a zombie up with a metal rod in him. After the zombie gets damaged from the rod, pick it up and ram another zombie with it, and continue to fire at the torso. Make sure to keep sticking zombies with the rod, if you dont the boss will regain all its parts and you will have to start over, when it comes time to shoot the boss in the eyes make you you stick a zombie with the rod before you blast away.
You keep using the rod to destroy his legs, he will eventually fall down, like you've seen. You can go up and do a melee that blows him up, but that will just regen him. Stick him with the rod in the eye when he goes down and it will kill him instantly.
This last form gave me so much trouble. To beat him relatively easily, Start by killing almost every single zombie in the area. After the zombies have been cleared out, either shoot off one of the bosses legs (yellow glowing segments on legs) or shoot the yellow sacks on his belly. After he is heavily damaged, he will attempt to absorb the nearest zombie to repair his limbs. If you killed just about every zombie in the area, then he is forced to grab the zombie with the lightning rod impaled into him and thus gets struck by lightning.
After he is struck by lightning, the lightning rod is thrown into the back of the boss room. You then have to pick up the rod and impale a zombie with it and start the process over again (making sure to kill every zombie in the area leaving only the boss and the zombie impaled with the lightning so that he has to absorb that zombie to heal and gets zapped for doing so). After getting zapped by lightning 3 times, the boss collapses. Now all you have to do is pick up the lightning rod and walk over to the bosses face and press the action button displayed to stick the rod into the bosses eye thus ending the fight.
This fight is very tedious as the boss just loves to absorb every zombie except for the one impaled with the lightning rod and he can kill you very quickly with his cheap acid ball attack and his impaling attack. Just keep moving and never give up as after a while, he will go down!
Stupidity isn't a virus, but it sure is spreading like one!