First Encounter Mission 5:
*** Spoiler - click to reveal ***
Jester is not a difficult boss, though he can be annoying at this point. At the start of the fight, fire Ebony & Ivory at him to make him dance and tire him out (attempting to attack him with Devil Arms causes him to teleport away). When he stops dancing and teleports away, this means he's tired and you can begin attacking him with melee to do significant damage. After a few moments, he'll disappear and do one of three orb-based attacks: In the first, he teleports to the center of the ring and creates three portals. The orbs that emerge from them revolve around Jester in a circular motion in varying heights. Standing at the very edge of the ring is safest; you can side roll through the orbs that happen to approach you, and continue firing when it's safe. The second attack involves an orb portal that fires orbs at high velocity at you. They come in waves but do not remain on the map; you can dodge them by jumping or rolling. The last attack is the most unpredictable and involves two portals that spew orbs that bounce around the map randomly. You can roughly tell where an orb is by observing its shadow and dodging as needed.
After each round of attacks, you must tire Jester out again before attempting to use your Devil Arms.