Mission 92: Xion
Roxas regrets his decision to leave Organization XIII. A hooded Xion approaches him with sea-salt ice cream at the Twilight Town clock tower, where she reveals to Roxas that it is time she "play her part" and absorb him to further Xemnas's plans. After an intense battle, Roxas defeats Xion. As Xion fades away, she requests Roxas to free Kingdom Hearts, leaving him devastated, alone, and shedding tears of sadness.
Day 357 takes Roxas to Twilight Town. Go to the Station Plaza, and save for the last time. Purchase any needed panels at the Moogle Shop near the far edge of the area. Walk up to the tower and choose to head up. A cutscene will play, then the boss battle with Xion's four forms begins.
Obtaining three Challenge Sigils is easy, as long as know how to predict and avoid Xion's attacks. Thankfully the amount of hits you could take without affecting your score is good, so don't worry about making some mistakes, but try not to depend on it if you want to get all 3 Challenge Sigils. Also for phases 1-3 only, when Xion goes into the motions to start the barrier to be able to heal her HP, quickly attack her directly with your Keyblade before the barrier goes up, otherwise this might affect your overall chance of getting 3 Challenge Sigils.
This Challenge Mission should be easy with the Zero Gear and three Ability Units. Use Limit Breaks to your advantage as to get through the first three battles quickly, and use a Limit Recharge in the fourth battle. Know how to predict Xion's attacks and avoid damage as often as possible. Use Hi-Potions or Elixirs to handle any healing, along with Limit Recharges to help make it easier to use a Limit Break, particularly if you need to heal. Much like the other challenge, keep Xion from healing at all costs during the first three phases, otherwise withdraw and then restart the challenge if she succeeds as it compromises your chances of getting all 3 Challenge Sigils.