I have just 100% and finished my 2nd playthrough of the game's hard mode. Some tips for people who want to give the challenge a shot.
You can cheese the entire game if you luck into the golden gears' event. By continuing to pay 50 gold, you can roll into debt and get an infinite supply of items (as there's no limit to the amount of debt you can go, and nothing stopping the event)
You can undo mistakes and events by exiting to the game's title screen and clicking continue to restart the battle/event. This gives you infinite retries, and even works during hard mode at path choices, shops and bunny wheels. Just make sure your hp don't drop to 0 and restart before the end (screen transition) of the event/battle.
For a no cheese run, the meta deck is definitely a defense deck. You can destroy everything using the combination of the red shield and green shield artifacts (1st one makes so whatever leftover defense blocks you have at the end of the enemies turn, turns into aoe damage, while the 2nd one makes so your defense blocks carry over to your next turn, and doesn't run out). If you combine that with the triple shield guy golden card (doubles your current defense, makes it carry to next turn), shout card (makes a copy of your next used card) and purple impact card (gives defense = to your current discard pile) you win.
The blue sword and the 9 cost lighting golden cards are also incredibly powerful in any deck. The sword damages the enemy and gives you defense equal to the damage, and the lighting 45 total damage. Combining those with the Red (adds +4 damage to every hit), gray (adds aoe effect) or the other red gem (gives a copy of the current card at its current cost) makes for some massive damage and cover. Just remember to open your deck and drag the gem to the desired card whenever you get one.
That's it. Incredibly fun game, with beautifully animated and detailed pixel art, and no AI bullshit. Go support the creators if you can