Ikenfell

Secret/optional boss. He only appears very late in the game (Chapter 8) when "cracks" start to appear all over the map due to the events in the final mission.

Enter the library, and go through the linear path until you find a grey book lying beside a crack in the floor. Touch the book to initiate this fight.

You can enable Instant Victory to beat the boss without fighting to unlock the achievement.

Codexus is an optional boss found in the Library. Near the end of the game, you will head to the top of the Spelltower and be shown cracks forming around Ikenfell. One will form inside the Library and knock Codexus off the shelf. Back track to the Library basement to challenge Codexus. You can activate the Instant Victory in the options and then select Instant Victory when it is your turn to defeat the boss.

Fasting way to level up and tip to get power to 100

If you still need to level up travel to the tree at the bottom of the final tower, walk inside the tower and in the first room theres combat in seconds, 3-4 lanterns netting 120-200 XP, then exit the door and re-enter respawning the lanterns. Walking in and out of this room and using auto victory is the fast way to level up in the game.

Tip to get power to 100

When playing to get the last gem inside the fighting arena / dojo, where you push down the 6 stones (needing 100 power levels) heres a tip.

if you were playing auto victory like myself and have already beat the game and don't want to grind to level 100 or are level 24+ and power is only rising in increments of 1+.

Don't forget you can us clothes to boost it!

Go to "equipment" and pick clothes that give you power, you can hit 100 power with clothes at only 65

A full achievements guide.

Click on the video and it will jump at Codexus's section.