How do you you perform fatality for raiden mk?

Well, if your talking about Mortal Kombat 9 you must go on too your moves list on the pause menu, go on too special moves and scroll down too the bottom. That character should have at least one...

you have to use combos and have to press specific buttons

You Kill 10 people without dying, then you get a special power called Adrenaline, press X Y Y, and viola...you can kill people in one hit!

Deathstroke: Foward, Down, Back, Fowrd, Down, Back, then circle and x together.