Thank you so much for playing!
Something I probably didn't get across as well as I wanted to in the game descriptions is that it is supposed to feel totally hopeless and frustrating between the time investment per attempt, close to unfair difficulty of the bosses even if you're prepared, and opaqueness of ways to improve your strategy. I did this to tie into the story, make it feel more rewarding once you finally figure things out, and hopefully encourage people to share strategies rather than go it alone (unfortunately barely anyone has bought the game).
That said I'll try to give some advice. For the earlygame, my recommended target to be on pace for winning the game is to reach level 4 within 15 minutes, ideally 12 or less. Midbosses or higher tier enemies are the most EXP efficient, so it's best to use your resources in ways that target them. Gold efficiency in this part is definitely tricky... slimes are safe but give terrible gold drops, while the tougher enemies give solid amounts but you can only snipe a couple with spells before needing an MP refill. Because of this, saving up gold is really slow, so you'll only ever afford any of the big purchases when you get a big influx of cash (your starting gold, overworld rewards, reselling your gear). You'll just have to make those sources count. Other than that, you can also save on healer fees by loading up on cheap items (usually health potions) and taking a death to make the duke heal you instead.