The boss of the Eastern Palace is Yuga himself. This battle has two main phases. In the first phase, Yuga will transform into a painting and move along the walls, occasionally summoning Popos or firing energy blasts. Your primary strategy here is to wait for him to emerge from the wall. When he does, quickly hit him with your sword. He will then retreat back into the wall.
In the second phase, Yuga will start to create paintings of Armos Knights that come to life. You need to defeat these Armos Knights using your Bow, aiming for their eyes. Once the Armos Knights are dealt with, Yuga will again emerge from the wall. This is your window to attack. Repeat this process, dodging his attacks and striking when he's vulnerable. After enough hits, Yuga will be defeated, and you'll rescue Seres, obtaining the Pendant of Power.