As title states. Regardless of whether you'd beaten D250 before, you'll have to beat it again to unlock access to the new D300 dungeon.

EDIT: As per this post - DeNA has officially recognized it as a bug and will be working on resolving it soon. If you're not keen on burning 60 stamina for this, hold off for now.

Also, only the VII one is available this time round. No idea what happened to the others.

Preliminary notes (many thanks to /u/Zurai001 for these):

• there is no trash
• mastery conditions for the fight is the same as before
• everyone starts with 3 bars of SB. No further bars can be generated. Abilities or RM that affects generation of SB gauges do not work (Lifesiphon still deals damage at least).
• Gi Nattak is only weak to Wind in his very weak phase. Weak phase is at 60% HP and Very Weak phase is at 30% HP respectively.
• Gi Nattak can drop 3x Major Black Orbs and 3x Major NE Orbs (credits to /u/CareerSMN and /u/robm1052).

Link to /u/Zurai001's AI guide