At the beginning ot the chapter you will still be in the Tournament town. To proceed in the plot you will have to speak to Etna in front of the fountain. Save beforehand, as there will be a battle against Yukimaru soon after.
The entire field will be covered by an Evade Geo effect. Yukimaru herself isn't too powerful compared to what you faced up until then, and bringing a high-SPD character can actually end the fight with them dodging every hit and pummeling her with counters. Otherwise, just whale on her with Hit-based weapons.
Afterwards you will be back to the hub, and have unlocked the Fifth Fight in the Battle Arena area.
Stage 6-5: Fifth Battle
Four squares each with a different bonus and a heaping dish of Enemy Boost inbetween. This stage is easier than it looks, and even the Rangers are pretty much harmless. If you are so inclined, do a chain throw at the beginning to sneak behind Red and destroy the Enemy Boost Geo Symbol, then lure the others out of their power zones for easy killing.