I'm sure you remember the blade I showed you. ["Showed"? He held it up to his throat and threatened to kill him unless he allowed Eren to morally interrogate him.]

It's attached to a special type of mobility gear that allows one to propel themselves to the titan's nape. The nape is then cut. It's the only way.

[An extremely simplified explanation straight out of his first year cadet textbook, but it isn't untrue, necessarily.]

That being said, some are harder to kill than others. [Like Eren, for example.]