Hyperdimension Neptunia Mk2

Delphinus is a Level S coliseum battle. To unlock this battle, you must first beat the game and start a New Game+. Once you reach Chapter 5 in New Game+, the coliseum unlocks and this fight will be available. It is best to attempt this battle with characters at least in the mid 90s. A party of 4 level 99 characters with high HP and AVD should not have much difficulty with this battle, though bring in many healing and reviving items, because she tends to hit hard. Having high AVD stats helps too, since her attacks will miss more often, so bring AVD boosting items, like Golden Feathers and White Owl Feathers. White Owl Feathers are better since they also raise an ally's TEC, so that you can land more hits. Other recommended items are P. SP Charger 1 & 2 to increase characters SP (especially Neptune's) so that you can use skills more often and down Delphinus quicker.

The best characters to use for beating Delphinus are Neptune partnered with Nepgear in the back row (be sure that you have their combination attack Violet Schwestern) by doing all of their Chirper events), and IF, due to her natural high AVD. The other two characters do not matter much, except do not bring mage-like characters (Compa, Gust, Ram, and Rom). I personally used Noire and Blanc (high natural HP). Spread out your characters so that she only hits one character at a time. To do a lot of damage, use Neptune's combination attack Violet Schwestern for massive damage.

Due to this being a coliseum fight, the game will not end if you lose. The only things that you lose are time and whatever items you used in the fight.