I think that you should emphasize more that time stops when you aren't moving. I haven't noticed it until stage 3. I thought just mana didn't recover while standing still. I guess it's natural to try and keep moving or attacking while there are enemies on the screen, so I didn't have a chance to see it working.
I'm not the biggest fan of melee and magic attacks using the same resource. I would even reverse it, so that melee attacks (or kills) recover mana and magic attacks use it up. So that you have to balance between using the two.
Also I think you give the player too many items. I had to choose like 3 items before I even knew what exactly was I doing. I think even one, but stronger item per stage would be enough