Here is an easy way to grind on a training dummy afk:
You must play in windowed mode to be able to do this.
1. Put the skill that has the fastest cooldown in your hotkey slot 1.
2. Go attack the dummy using that skill by holding the hotkey you put it in(1 for example).
3. While holding the key down, minimize the game window.
4. Go back to the window and see your character repeating that action over and over.