1. Check the marketplace everyday to assess the prices of end-game materials (e.g. Godly Ores, Heavenly Leather, Godly Cloth, etc.) Once you're comfortable with the prices, check if there are shortages of a certain material and take advantage of supply and demand.
2. Assess events and turn it in your favor. Depending on events (like the Ancient Coin event where we could exchange coins obtained from raiding certain bosses and recieve an enchant scroll of our choice) it was reasonable to think that blessed elixirs would be running low on the marketplace, leading people to farm Two Moons and craft said elixirs to sell. Easy profit.
3. Daily s2/3 raiding + Muir and Succubus.
Daily raiding may yield some enchant scrolls here and there as well as OJ drops. Running Hero Mode Succubus (Secret Naughty Chamber) will give you a small chance for an Enthus ES or 2 other enchant scrolls that aren't as profitable. She also drops enhancement elixirs so that's a plus. Muir and his break off (tail) will provide a couple paradise stones a day.
4. Daily Niflheim. On top of removing the attack cap, they also made it too easy to obtain the respective keys by adding a death cap of 3 per person. Experienced parties can finish each run in 5-8 minutes -- or even less. Exchange the keys for the superior bracelet gem boxes and if you're lucky, you'll get a superior gem (took me ~243 boxes for a superior emerald gem).
That's all I can get off the top of my head while on my phone, for now. Might add more tips if I feel like it.
At this point in time, there's no alternative to making a decent sum of in-game money. Of course, the amount of in-game money someone makes compared to another may seem insignificant depending on the person.
Your best bet for making stable income at this point in time is to farm Season 3 dungeons for the materials (Eg. Godly Cloths, Heavenly Leather, Godly Ore, etc.) that may increase in price for the future smash update where we'll be able to min/max our stats and hoard them.
Doing daily runs such as Rocheste By Sea (Muir) for fast and easy paradise stones (don't forget B/O) and Hero Mode Secret Naughty Chamber (Lv. 50 Succubus) for a small chance at an Enthus ES (valued at 25-30mil) are other alternatives as well.
Farming rings are somewhat worth it depending on the ring itself; if it's a CMA the value is about 60-80 mil but if it's a TR it's around 20-25mil.
If you're looking to make a large sum of money, I currently see no option but if it's a stable income you're looking for, perhaps the method mentioned above will prove useful.