Sea Salt

There's a beneficial exploit when playing as the Hunter (unlocked after your first normal playthrough). Anytime you increase his attack and then pause and restart the room, the hunter will keep the powerup but the Summoning Circles will reset and can be used again. Doing this you can spend a few minutes increasing your attack in the first room with a summoning circle until you are strong enough to 1-shot most enemies. I did this to power up attack about 20 times off the first available circle and it made the rest of the level very easy.

Hard Mode is a cake walk if you use The Hunter (gotten by watching the credits and waiting for a bit on the "THANKS FOR PLAYING!" screen and playing until you get into the Cathedral). Level up Attack every chance you get. Save your money as a 'health kit' when you need it; if you get low, just hit and buy the Health card. If you have your money capped and there's a bunch on the ground, just level your Attack and pick it all up. Also, you still can retry a level as many times as you want, so don't worry about dying. If you need any specific pointers, just make a comment and I'll edit this with more info.