"I'm Rob Zombie: Musician, Director, and... forget about it."
Before you can even get the chance to fight Rob Zombie, you have to collect the special pieces found throughout the levels.
Specifically, the levels with these pieces are:
- On the Beach
- The Sewers
- Phone Home
- Tower Power
- Of the Dead
- Rock-it Ship
- Rendezvous
- Hard Rock
Once you find them all, (when you get one, you'll hear a voice) you'll unlock a bonus level called "Iron Head." This is the "uber-secret level" where Rob effing Zombie is the final boss.
And BOY is he hard. You have NO continues, NO power-ups, and extremely limited health. Unless you have a death-wish, DONT try this above Normal difficulty. I don't know if you can do it on Easy, but I did it on Normal and it wasn't THAT tough.
The only way to get health is to destroy certain objects. When you're facing the "coffin amps" (After NPH says "whoa, epic!"), you see what looks like crystals sticking out of... something. When you get the chance to destroy that, DO IT. I think there are 3 of these in the level.
Also, when you see the "zombie dancers" (And Rob Zombie says "HEY, LEAVE THEM ALONE!"), take them down for bonus health as well. This comes immediately after the first giant boulder attempts to flatten you.
Other than that, it works the same way as the rest of the game. Once you beat him, you'll get the achievement and... nothing else. Bit of a bummer, considering the "SUPER SECRET" nature of this level.
UPDATE - Beat it on Thrasher difficulty. Other than a sense of self-accomplishment, I gained nothing for it.