When you have the indicated amount of Renown you will gain instant and free access to the following items after you have cleared Chapter 3. Renown is earned by completing non-DLC battles (10 for Story or Skirmish, 50 for Spotpass or Streetpass battle, and various amounts for Double Duel). Items can only be obtained once per file and Renown is not spent when they are obtained. To carry over Renown to new files you must clear the final chapter.
To make any unit supremely overpowered, get them to level 20 (30 for dragons and beasts), then use a second seal on them, but make them the same class. It will reset their level to 1, but keep their stats the same, so you have 19 more levels to train your favourite units. Enjoy!
Collect a large number of stat increasing items and gold and then save the game. Recruit a character from the Avatar Logbook and apply as many stat increases as you want to that character. Update the avatar logbook for that character and then create a new saved game. Load the first saved game and keep repeating the process until stats are maxed.