Mission #55 rewards you w/ the accessory "Grow Egg" (2x CP for All Characters). Obviously, this is an incredible item, but the battle itself is far from easy. Thankfully, there is a "cheap strategy"...
Leader: Vanille (Nirvana or Malboro Wand, Dash Shoes)
Member #1: Fang (Dash Shoes)
Member #2: Hope (Dash Shoes)
Optima #1: Jammer/Defender/Healer (Default Optima)
Optima #2: Healer/Defender/Attacker
Optima #3: Healer/Attacker/Attacker
Neochu isn't immune to Vanille's Death, and we will use it to ensure a swift victory. She *must* have Nirvana or Malboro Wand, and if at all possible, 2000+ Magic Stat. Power Smoke, Barrier Smoke. When the battle starts, spam him with Death until he dies, then switch to Optima #2 or Optima #3 and clean up the remaining enemies.