Thanks a lot for your feedback!
Monsters not regening while stunned is intended, but I have noted that the stun in timed mode is certainly too strong.
Currently, timed mode enemy regen is operating on the same rules as classic mode: where enemies reset after a missed turn, rather than on a time-basis.
This was set to not make timed mode "too easy", but I can see how it would be unintuitive. I believe what you were encountering might've been a result of the stun timer, which does operate on a frame basis, ending as your combos were ongoing. I will make sure to look back and check for bugs though in this area.
The new classes currently planned are harder versions of the base two classes, with access to different abilities and items, and unlocks. That being said, more might be added as I come up with ideas.
An update will be coming to this demo soon, with a new level and the unlock/equipment system. Thanks for playing, glad you enjoyed the game!