Now come to the most boring part of the game. Fortunately, you don't need to maximize everything in order to get 100% achievements. The most important resources that you want to prioritize when grinding are Titan Bloods, Ambrosias, and Diamonds. 

First, I would recommend you to complete the story at least once (defeated Hades for the first time) to unlock Pact of Punishment. Once you unlock it, your grinding speed will be faster. The reason is that whenever you increase the Heat (difficulty) through the Pact of Punishment, the first-time reward will be reset and you can obtain Diamonds, Ambrosias, and Titan Bloods again. Each weapon has their own Heat level, that means when you increase the Heat, you can obtain the first-time reward 6 times with different weapons.

The Fated List of Minor Prophecies is like the in-game achievements which you complete certain tasks in exchange for rewards. 

Reward: 

Diamond and Titan Blood can be purchase using gold at the Temple of Styx (4th region). I usually save up Gold to purchase it every run because they are expensive:

• 1 Diamond = 1,000 Gold
• 1 Titan Blood = 1,200 Gold

Only either 1 Diamond or 1 Titan Blood is appeared per run, so you only need to save up 1,200 Gold for the purchase.

Beside completing The Fated List of Minor Prophecies for rewards, the best way to grind Nectars, Gemstones, and Darkness is through running the game repeatedly. You should prioritize whatever currencies you would like to farm when multiple options are present.

Note: 

• Prioritize purchasing Work Orders from House Contractor will help speed up the grinding. For example, Plunder, Superior will increase the value of trove,  Vanquisher's Keep will drop additional Gemstones after defeated a boss etc. 
• Use Fated Authority from Mirror Of Night to alter rewards for the next chamber. For example, if you see Darkness is the reward, you can use 1 dice to random change the reward to one of the 4 (Nectar, Chthonic Key, Gemstone, or Darkness). This is useful if you want to farm Chthonic Keys or Nectars faster because you can keep alter the reward until you find them.
   

It appears randomly through out the regions, so you should watch out for it for additional rewards. When you open the trove, you have to kill all enemies as soon as possible to keep the value in the trove high. The Infernal Trove contains the following rewards:

• Gold
• Darkness
• Health
• Gemstone

Fishing is another way to get extra cash. But first you have to unlock the fishing ability by purchasing Work Orders "Rod of Fishing" which cost 1 Diamond. you can learn more about fishing from Fishing section. After you catch fishes, go to the Lounge and talk to the chef, he'll give rewards in exchange for the fishes you caught, The rewards vary from Darkness to Titan Blood.