Great game, very well balanced, and excellent content. Only thing I could wish for is a save system so I could continue my endless mode run over more than one session.
Here is a thorough guide to obtaining an S and a score of just under 10,000 after the lvl 25 boss:
Priority: upgrade two defence, one attack, and one boost to max. Get grow on at least one die (preferably attack and either defence or heal).
After that (or if you're not getting upgrades), discard the leftover attack and boost dice.
Only get a new die if you've done the above/haven't got anything else to do! (your goal is to minimise the randomness of your turn.) If you do get a new die, only non-glass heal (skip if you get anything else). After that, discard the glass heal.
Mirror die is great to get (not a must, but makes boss fights much easier).
Make a couple of them hollow, so you can use practically all your dice in one turn.
During your turn prioritise defence, end each round with max health, and during boss fights just wait it out, getting your grow dice to higher values.
And if you complete all the above, you will feel like a god on the final boss (I destroyed both its phases in 6 or 7 turns).
Hope this helps anyone!