Enemy mana was floated as a possible thing during the alpha days, but it didn't really add much to the game and just made predicting things more complex - like, we could have achieved exactly the same thing if we'd given enemies alternating attacks every 2 or 3 hits, etc. In the end, there was more than enough complexity in the game, so adding enemy mana seemed more like it would just be ways to screw the player over than anything else. Player mana is the great equaliser, after all