[Chazar]

I think that retreat should just work as usual during assassination:

However, for an assassination, all troops should just be placed on switched sides (i.e. victim on the left). So if the victim wants to retreat, he can do so, but he must cross the whole battlefield in order to escape into safety...

If the assassin is too stupid to kill the poor victim over the entire distance, well, his own fault!

(Hmm, might still have a problem with fliers and mounted commanders, though...)