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This mission has the player character facing hordes of Unversed which are harder to defeat than there were in the Dead Ringer mission, before finally rematching the Mimic Master. It is recommended to go into this mission with a screen nuke or two: Mega Flare, Meteor and Glacier are especially good for taking out the Unversed hordes. If the player is entering this mission as Ventus, Faith is a great help: it can damage enemies while healing him. However, do not stock the player's deck solely with screen nukes: keep a Curaga or two handy, and keep a Sonic Blade and a few Firaga commands (or Triple Firaga, if playing as Aqua) just in case a homing attack is needed.
The Mimic Master is more relentless with its attacks this time. Start with a rushing Shotlock, e.g. Photon Charge or Absolute Zero. Make sure the Shotlock is at its maximum level, to get the most use out of it before depleting the player's Focus gauge. The Mimic Master can be hurt when it cloaks itself in fire and travels round the arena, so quickly make an opening to hurt it.
When it shoots beams of light and darkness, stay aloft as much as possible. Jump with the correct timing to avoid it. Aqua is at an advantage if she has Doubleflight equipped in her deck. Casting a Barrier or two will make her stay aloft for longer. Dodging into the beams of darkness does not make the player invulnerable to them; they will be made blind for a period of time.
As the Mimic Master has more HP, it may create copies of the player character more than once during this battle. If the player is using Ventus, Superglide is the best way to avoid being sucked in and copied. Ready the rushing Shotlock to take the copies out, and use Firaga, Triple Firaga and Sonic Blade as much as needed. Heal where necessary, too. Renewal Block and Renewal Barrier are invaluable for this section. It is possible to use a screen nuke to damage as many copies as possible—Meteor is a good command for this purpose—but most screen nukes only do minimal damage to bosses, and these copies are no different.
If the player character is Terra, a good way to take out the copies is by sliding and following up with Sonic Impact. Terra's copies will also use Sonic Impact; however, he can hurt his copies by using Sonic Impact relentlessly, and they will still be hurt upon taking damage while using that same ability.
With enough patience, the Mimic Master should be defeated.