Well you can progress for free with some patience, especially now with the new gems all you have to do is keep rebirthing asap and everytime you will get more gems of luck. I still dont know if there is any reason to progress though... i only got to 120 so far but i didnt get anything that I couldnt earn easier in the lower levels. 100+ bosses stopped dropping the normal red gems for me, dont know whats up with this. In the normal version you can still farm them by killing the same 100+ boss over and over again. Afaik this never worked in the nutaku version so maybe their new gems broke it completely.