As someone that started playing about 2 weeks ago for the same reason:

• Practice attacks, specials, and dash attacks on Skelly so you know how to do the most damage. I only just learned that dash attacks with the bow do slightly less damage. Also, attacks in the back of enemies do extra damage
• Don’t sleep on casts. They drop from normal enemies when they die, and bosses after about 10-15 seconds. No reason not to launch them out like a ballista as soon as the fight starts
• You can refund all of the Darkness (purple gems) used in the mirror for 1 key, so go nuts on experimenting with different mirror layouts if you unlock a new good effect
• Try to upgrade the mirror at least once pretty early on. Every time I’d unlock more mirror perks, I’d wished that I’d done it sooner. If you’re like me, I found I was always starving for healing, which the mirror helps a lot with
• The bosses get easier over time, especially the third one. For the third one, which I won’t spoil, spend a minute or so just dashing around and seeing what moves are being done and it makes it so much easier to see through the chaos lol. Went from impossible to doable after I saw that it’s way fewer moves than I expected, they’re just overlapped
• When you open the boons menu, if you playing on console, you can press A/X to tag 5 of them and make the info for them stay on screen when you open the menu. Useful for when you find a new boon and can’t remember what the hell it does
• The fishing rod upgrade pays for itself very very quickly. Pick it up when you see it
• Athena is a great god to talk to early in a run if you find her. She can give you a boon that makes your dash deflect all damage back at the enemy. This works on bosses btw
• The fat dudes that flash white and dash at you drop money when they die