Here is the information for level 11-16 of Ruins of Yellow River in Atlantica, have a look as that can help you more or less.

In order to get past level 20, you must finish Greetings by Sima Qian. Make sure you finish that quest first even if you despise questing. The Headless Warriors, Soldiers, and Punishers are a nice place to grind. They have decent drops. The moment you hit level 15, try to buy some Sea King Equipment Box or just Ocean, Deep Sea or Red Soul Equipment for your team. It helps to make a lot of the boss battles easier. If you rich enough, that you can buy high upgrade versions of the equipment and use them for many levels to come. A +3 will last you until level 35.

Margo's Evil Archer

Margo's Evil Archer is the mid boss of the Ruins of Yellow River. You can party with other players against it. Its a giant sized version of the other archers in the area. Its fairly strong and attacks 2 to 3 times regularly. If you're a ranged or staff main, this could kill you in 1 turn. Again, use 2 Vortex scrolls to clear out the minions first.

Xiyu Shaman

The Xiyu Shaman is also the boss of the Ruins of Yellow River. Its easier than Margo's Evil Archer because it wastes turns healing the minions. Try to get off Vortex scrolls early on to clear the minions so you can focus on the boss. As long as you can deal damage to it regularly it won't attack you directly.