Hardcore mode in this game is incredibly difficult, and probably was never intended to be beaten, so it’s curious that they added an achievement for just that. Some exploitation is basically required to beat it in a reasonable amount of time, so we’ll be doing just that.
Start out by fishing, and clear the first level or two using your pet with a powerful transformation. You can find some legendary (yellow) items by fishing, which will do fine as early game equipment, and some of them sell for a lot of money (up to ~400k gold).
If you’re going into a magic build like I did, put stat points into vitality and magic, and all skill points into attack magic. You can put some points into spellcasting much later when you have enough mana regeneration to cast indefinitely. Start using summons to help your pet fight, but don’t put any skill points into charm magic because summons scale very badly.
If you’re not going into magic, the early game will be a little harder since you probably won’t have the magic requirement for some of the better summons. Put points into Strength, Dexterity and Vitality, and skill points mostly into your weapon skill.
Making Money Later
Once you have aquired some level ups, you can stop fishing and instead start making money with enchanting. Find or buy expensive nonmagic items from a vendor, add an enchantment or two (ideally a unique enchantment) at the enchanter and sell it for a profit. The enchantments you get are based on your character level, so the higher your level the more money you can make. Later on, you can make money very quickly using this method.
Crafting Ridiculous Items
Once you can start making money with the enchantment method, you can start crafting items. Buy the cheapest nonmagic item vendors offer (cheaper base items make enchanting cheaper), and start enchanting it.
If you get a good enchantment, save & quit and then continue the game. If you get a useless or bad enchantment, terminate the game in Windows and start it up again, reverting that enchantment. This exploit is time-intensive, but you can create ridiculous items with it.
For a caster build, you’re mainly looking for Magic, Vitality, Mana, Health, damage reduction, block chance (use a shield), faster casting, mana regeneration and magic skill points. Fill any sockets with mana regeneration gems until you have enough, then switch to faster casting.
The level of enchantments you get depends on your character level, so keep progressing in the dungeons between enchantments to keep improving your gear.
If you spend enough time enchanting using this method, the rest of the game should be a piece of cake. Please upvote the guide if it helped you out.