A lot of the monster content 'unlocks' with high level generals who have also been promoted. A good example of 'unlocking' monster content is Cronus Astaroth, where you need to do 102 million damage in order to get 100% drop on the legendary fiery blood vials or icy blue vials. Level 4 generals just don't provide enough of an mAtk boost or a damage boost from stats to farm Astaroth effectively. This tiny guide lists the basic tips you should remember when trying to farm the crystals needed to level up your generals.
In order for crystals to drop the monster must also drop essences of any kind. So, you can hit until your face is blue on monsters like Genesis, Ragnarok, and War of the Red Plains and nothing will drop, because these monsters don't drop essence. Also, red crystals only drop on Sundays and Mondays, while greens only drop on Fridays and Saturdays. (California time.)
The mechanism behind colored crystal drops from monsters and from excavation quests is most basically, 1 red crystal per 600 stamina invested, 1 green crystal per 800 stamina invested, and 1 green crystal per 2000 energy invested. That's it. There are surcharges for starting off, where essence drops at 150 stamina, but this lessens to essence every 120 stamina with increased investment. This means, in order to maximize your investment, that you should hit THE SAME MONSTER when farming crystals, rather than spreading your stamina across several monsters. Of course, you need to balance this with loot returns on stamina investment, but monsters like Rats or Astaroth help you out when you farm crystals.
In order for you to gather enough colored crystals you have three options. 1) Have patience, because over 4 days of crystal drops you can only spend 1200-2400 stamina or energy through natural regeneration you will net only a few crystals per week. 2) Refill stamina and/or energy, the downside is it costs hard earned real-life currency to purchase favor points, or real-life clicking effort to farm serpents for favor points. Farming serpents is, though tedious, effective and many F2P do it religiously. 3) Adjust your build to a high LSI build. The example I have in mind has over 10,000 energy and 2,600 stamina on level up, and levels up once every 30 hours with natural regeneration, or once every time the toon refills. Throw in Solara and Hyacinth for reds and greens and you get 50 coloreds per week.
Blue Crystals are a beast of a different color. These drop when you spend conquest tokens in Land of Earth, either by attacking someone else's land, or by defending your guild's land from attackers. The drop rate of reds and greens is fixed at 20% and 15% per excavation or essence drop, but blue crystal drop rate is highly dependent on your guild's conquest rank. The key to farming blue crystals then becomes simply being in a guild of a high enough level. Conquest tokens are essentially 'free' resources, and regenerate over time, so blue crystals are by far the easiest of the colored crystals to get. Expect at least 15 blues per week on natural token regeneration in a rank 24 guild.
That's just about it! They key is to avoid frustration by knowing that it takes hundreds of thousands of stamina to level up an aggressive general to 80, and just to have patience, or be willing to spend to refill.
Good luck and have fun!