A great place to grind for money is in the Templo de Osidiana. When taking the upward path you'll encounter the Summoner enemy in an arena fight. This enemy will keep summoning the larger skull monsters until you defeat it. Ignore it and just keep defeating the enemies it summons. Note that you won't earn any money from defeating these creatures,
only from boosting your hit counter. With that in mind I highly recommend that you have already bought 'Gilded Gold Hits' from Flame Face's skill tree. I also recommend 'Tough Guy' so you can get hit a few times and not lose your streak.
***IMPORTANT*** You can't gain health in this fight by normal means. If you die, you'll lose
ALL the money you've earned and any skills you may have purchased. However, if you buy an upgrade it will completely heal you. Keep some upgrades available for purchase to avoid death.
I've provided a quick video showcasing how this fight should work. Note that I have already purchased a lot of upgrades so I make quick work of these guys. I'm also using Uay Coco as he uses half as much Stamina for his super moves, but he has much less max health so it's a bit risky. (I'm playing on Hard Mode as well so enemies hit harder) Once you get a rhythm going it's easy, although a bit tedious. You can always cling to the wall in the right corner for a break and to recharge your Stamina.
Your hit counter maxes out at 999 but you can keep going and still earn more gold.