There are two methods I can think of off the top of my head:
1. Split the enemy vertically into two sprites(left and right sides), one half above the sword, the other below it.
2. Use a separate sprite for the blade. Using some basic maths, adjust the length of the blade such that it doesn't extend beyond the horizontal center of the enemy sprite.
Number 1 is the simplest in some respects(no math) and allows the blade to appear out the other side. On the other hand, splitting the enemies like that could complicate other things. Neither one will be truly simple, of course.