Successfully complete the game and save when prompted to unlock New Game +. Start New Game +, then successfully complete the second Varl Mountain event to enter a bonus dungeon.
Successfully complete New Game + to unlock New Game ++.
Successfully complete the game to unlock the Hard difficulty setting.
Successfully complete the game on the Hard difficulty setting to unlock the Very Hard difficulty setting.
If there is a high ledge that you could not normally reach without a Selkie or one that is slightly too high for even a Selkie to reach, just drop Magicite from your pockets. Pick up each Magicite and pile it on top of one another. Be careful not to throw the Magicite or the one on the bottom of the stack will detonate. Three or four Magicite stacked on top of one another will allow a Clavat, Yuke, or Lility to reach a height a Selkie could make in its double jump. You can also use the Selkie's double jump on top of Magicite to reach even higher places.
A Lility's major downfall is that it has a combo of one. Its slow attacks allow it to get pummeled easily by a gang of enemies. When you are a Lility (Meeth for single player but whatever you name it in Multiplay), hold the D-pad in the direction that you are attacking. For example, if an enemy is to the right of you, hold Right and keep pressing A. You will attack a lot faster than if you just stood still and fight the enemy. You will not be anywhere close to the attacking speed of a Clavat, but it will be better than what you normally could do by standing still.
When you go to Rebena Te Ra, buy out everything in the Mog Shop. To get to the Mog Shop, go up the steps leading to the Crystal Temple, but do not go in. Instead, jump in the upper-right direction before you pass the guard. You will see a Moogle at a shop stand. This is the Mog Shop. Buy out everything (all the HP, SP, ATK, DEF, and MAG boosts; and all the pockets and pouches). Once everything is sold out, do a quest. When you complete the quest, check the Mog Shop again. There should be more items in the shop. If you run out of quests to do, complete an easy dungeon, such as the Valley of Heroes. Keep doing this as many times as desired. The Mog Shop will never run out of items to restock.