The most essential tip for this game is to stick and move at all times. Keep running and never stay in range of enemies any longer than you have to in order to get a couple of slashes in.
Bosses all have patterns you can exploit if you learn them. Watch for openings, then go in and get in your licks. It helps to enter with a full mana meter. Unload all of your magic to start the battle, then make smart melee attacks to finish them off.
Don’t leave home without some potions. At the same time, don’t waste too many coins on potions. Permanent upgrades are always going to be better. But keeping one or two potions in stock can sometimes make the difference between finishing a stage or failing it, and that’s worth the handful of coins they cost.
Destroy every object you can. You’ll earn extra coins and sometimes find a very valuable health or mana refill. It all adds up.
Be extra cautious around traps. While you can’t help but take some damage from enemy attacks, you can always avoid taking damage from traps if you’re careful enough.
After finishing a stage, it’s a good idea to just let yourself die repeatedly at the beginning of the next stage so that you can start it fresh with a full stock of lives.
Mana refills are far more plentiful in the stages than health refills are. Use your magic often as it allows you to take out enemies from a safe distance. Most bosses will have a mana refill station somewhere near them so you won’t need to go in empty-handed.
For similar reasons, it’s better to buy permanent upgrades for your health before investing in increasing your mana bar. A bigger mana bar will give you more free shots against the bosses, but more health is useful in any scenario.