Nice concept! There's a good base for an automatic RPG. Here's some thoughts :
- The damage could be automatic with a dps system (instead of going back and forward with your sword).
- Enemies movement are a bit hard to predict. A little anticipation feedback (ex: arrow direction) may help.
- I like the upgrade system. It's fun, easy to use and give a good progression feeling.
- 10 seconds penalty has no impact for now. It's because the time does not matter. You can add ranking, par or medal system to challenge player about their score.
Good job by the way! :)