It's not really impossible, just extremely extremely hard. Made in part by the fact that yes, Raiden seems to be a pretty craptacular fighter compared to the rest of the characters you play as. His combos are slow and limited, his specials are weak and ineffectual and even his x-ray seems underpowered.
Also, Kahn fights as cheap and dirty as he did in MK2/3 spamming his most powerful moves over and over and over and over and over again until you are dead, making a fun and dynamic fight impossible. In order to beat him you have to resort to the same cheap BS methods. Kahn seems pretty vulnerable to kicks. Jump kick and kick-combo him and just spam whatever keeps him off balance until you win.