Xion does a cycle of 3 turns of regular attacks, and then a 4th turn where she does Keyblades into a Cleave.
After you've taken 5 lives, she will have a chance to counter with the Laser projectile on any attack that does not take a life.
After 7 lives, she adds a 4th attack to her regular attack pattern.
After 11 lives, Xion's next turn will be setting up her Ultimate attack. She will FP Buster, Cross Cleave, and Black Hole. It consists of a barrage of Lasers, most of which need to be blocked in order to survive.
After her first Ultimate, Xion starts a new cycle which consists of 3 turns of regular attacks, Keyblades, 1 regular attack, and then Ultimate again (starting with FP Buster, etc.).
After 13 lives, Xion adds an all-party magic attack to the end of her 4 regular attacks, and at the end of Keyblades + Cross Cleave she will throw a Laser. This puts more emphasis on needing to block well to survive.
After 14 lives, Xion is guaranteed to counter any attack that doesn't take a life.
Strategy
It's recommended to do 15k damage with each attack due to how Xion can counterattack. With at least 600 magic attack, Jump and Geno Whirl will do at least 15k damage.
If Mario has Super Boots equipped and at least 600 magic attack, he can do 30k damage with Super Jump. This is useful for preventing Xion's counter.
Dark Energy and Armageddon are useful for taking lives near the end of the fight and avoiding her counter.
Stocking up on Lemonade from Shy Ranger is useful as it allows your item user(s) to keep everyone fully healed.
Keeping your main healer at full health is very important. Life Shroom works well because they heal fully and give auto-life, which you will likely be using a lot.
Your whole party should be near fully healed on the turn she's going to set up her Ultimate. If someone dies from the Cleave it becomes more unlikely you'll survive the Laser spam.
You will likely need Kerokerocola, Galaxy Star, or Celebi to fully heal HP and FP after the FP Buster to survive the ultimate. Saving Celebi for the last few lives for a full heal and extra damage may be useful.
Xion's next action after her Ultimate is a 30k heal which recovers 1 Life. You can use this turn as a "freebie" of sorts to do extra healing if needed.
Always be ready to block her counters when you know she might use one. If blocked they only do around 1k damage and are much more manageable.
It is recommended not to attack with the party member who goes last if there is a chance Xion will counter. This forces you to react to whether she counters or goes straight into her regular attacks, which is quite dangerous.
The tricky part about blocking Xion's attacks is the delay required when a party member gets KO'd. The down animation on KO can throw off block timing, leading to a misinput and the next party member not being able to block. Take extra precaution to take note of everyone's HP before Xion attacks to get a feeling for when someone will get KO'd. If someone with auto-life gets KO'd, this gives you much more time to prepare for the next block, so a misinput isn't an issue.
Xion will destroy any Manifest Illusions that you throw out. Even if you have one active before the battle, it doesn't work.
There is a Hard Mode specific bug concerning Xion's regular attack pattern with 4 attacks. If the first 3 attacks are blocked, the 4th attack becomes unblockable. However, if one of the first 3 attacks is not blocked, the 4th attack can be blocked. (Has since been fixed as of Hotfix 1)
Xion does a cycle of 3 turns of regular attacks, and then a 4th turn where she does Keyblades into a Cleave.
After you've taken 5 lives, she will have a chance to counter with the Laser projectile on any attack that does not take a life.
After 7 lives, she adds a 4th attack to her regular attack pattern.
After 11 lives, Xion's next turn will be setting up her Ultimate attack. She will FP Buster, Cross Cleave, and Black Hole. It consists of a barrage of Lasers, most of which need to be blocked in order to survive.
After her first Ultimate, Xion starts a new cycle which consists of 3 turns of regular attacks, Keyblades, 1 regular attack, and then Ultimate again (starting with FP Buster, etc.).
After 13 lives, Xion adds an all-party magic attack to the end of her 4 regular attacks, and at the end of Keyblades + Cross Cleave she will throw a Laser. This puts more emphasis on needing to block well to survive.
After 14 lives, Xion is guaranteed to counter any attack that doesn't take a life.
Strategy
It's recommended to do 15k damage with each attack due to how Xion can counterattack. With at least 600 magic attack, Jump and Geno Whirl will do at least 15k damage.
If Mario has Super Boots equipped and at least 600 magic attack, he can do 30k damage with Super Jump. This is useful for preventing Xion's counter.
Dark Energy and Armageddon are useful for taking lives near the end of the fight and avoiding her counter.
Stocking up on Lemonade from Shy Ranger is useful as it allows your item user(s) to keep everyone fully healed.
Keeping your main healer at full health is very important. Life Shroom works well because they heal fully and give auto-life, which you will likely be using a lot.
Your whole party should be near fully healed on the turn she's going to set up her Ultimate. If someone dies from the Cleave it becomes more unlikely you'll survive the Laser spam.
You will likely need Kerokerocola, Galaxy Star, or Celebi to fully heal HP and FP after the FP Buster to survive the ultimate. Saving Celebi for the last few lives for a full heal and extra damage may be useful.
Xion's next action after her Ultimate is a 30k heal which recovers 1 Life. You can use this turn as a "freebie" of sorts to do extra healing if needed.
Always be ready to block her counters when you know she might use one. If blocked they only do around 1k damage and are much more manageable.
It is recommended not to attack with the party member who goes last if there is a chance Xion will counter. This forces you to react to whether she counters or goes straight into her regular attacks, which is quite dangerous.
The tricky part about blocking Xion's attacks is the delay required when a party member gets KO'd. The down animation on KO can throw off block timing, leading to a misinput and the next party member not being able to block. Take extra precaution to take note of everyone's HP before Xion attacks to get a feeling for when someone will get KO'd. If someone with auto-life gets KO'd, this gives you much more time to prepare for the next block, so a misinput isn't an issue.
