You get it when you first verse bizerker.

Yeah the Hammer is available whenever you encounter the berserker, except the last lvl on the train. Just disconnect the train car. thats the easiest way.......

Then, after U've PWNED it, just PWN it whit Ur rapid-blaster( didn't find the name)

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u get it every time you beat the berserker

you can get it just before the first seeder