@ Igor83185 ( in top comments) a good strategy for the final boss is to stay in his spawn area near the throne with the final greatsword and the knockback perk. Sit there and hit and the enemies will rarely hit you if you keep spamming attacks. It also helps if you have the full regen full greatsword damadge multipliers (perks and bracelets) and full defence. This should easily see you through the boss fight. A good strategy for the slow and long grinding in this game is to destroy first one or two and then leave 1 hit to destroy the next door so that you can stop enemies spawning when you become overwhelmed and just keep killing enemies.
I thought that this was a game made by someone not really well known who thought that people would sign up to other websites and then I realized who Iriysoft is! You made cursed treasure and countless other games. This would have been much better with a half decent sponsor.