Mission 85: Collect hearts
Eliminate Heartless in Wonderland and collect hearts.
Roxas goes to Wonderland to collect hearts.
The Mission Gauge fills to the goal line after Roxas collects 596 hearts, but why stop there? Continue through the Lotus Forest clearing out all enemies. A Lurk Lizard appears when the Tea Party Garden is clear. Defeating it is sure to fill the Mission Gauge to the max.
Just use Magic and regular attacks to defeat Grey Caprice, Striped Aria, Sapphire Elegy, and don't bother with Novashadow and Lurk Lizard. At least make sure to have both your Magic and Attack stat raised high to improve results and bring a few Elixirs to restore magic casts, along with a few Curas to help heal without wasting Elixirs. If you defeat every enemy you find (excluding Novashadow) before reaching Lotus Forest, then there won't be too many to face there before defeating enough Heartless to be able to use the Exit point. As long as you are prepared this will be an easy mission, though the only annoyance is Sapphire Elegy's teleporting ability to evade attacks.