How do you describe the methods that someone uses to achieve something no matter what would cost? Like Frank Underwood in House of Cards. Is it called Machiavellism, Diplomat or something else?

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I would probably describe this type of person and their methods as "ruthless" - because in this case I think what you mean is somebody who wants to achieve no matter what the cost to them or to anybody else.

Machiavellian might also work in this case, although to me it has a slightly less personal feel to it - like it's all just a huge game, nothing personal, but everybody is playing to win and playing for keeps, and nobody can be trusted. (Somehow the example of Frank Underwood to me seems to have a little more emotional content than I think of with "Machiavellian" - but this is really splitting hairs...)

Henry Kissinger and Zhou Enlai are probably best described as cunning and Machiavellian - practitioners of realpolitik. Stalin was ruthless.

If it's just a matter of achieving no matter what the cost to yourself alone - then that might be better described as single-minded.